I did some exploratory stats on this question from data at http://www.alltimeathletics.com/mdecaok.htm .
This data includes 2,407 performances of 8,000 points or more by 410 athletes.
The data shows that it is quite rare even for world class athletes to complete many decathlons in a year, with only 9 athletes finishing 5 or more high quality decathlons at least once in a season.
Of those 9 athletes, only Attila Zsivóczky hit his maximum (5 decathlons) in two different years, 1999 and 2005.
Over those 9 athlete's career, their mean annual performances ranged from 1.8 to 3.0 (Toomey and Sebrle).
Here is a table of the number of performers who achieved a maximum number of decathlons in a year:
Note: Only an athlete's best year is counted.
Decathlons Performers 6 2 5 7 4 37 3 87 2 112 1 165
Results 21 to 30 of 32

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04242019 06:58 AM

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04242019 07:17 AMHere is a bit more detail on the 9 athletes that did 5 or more in a season.
Athlete Max Mean Total Seasons PB MeanScore Roman Sebrle 6 3.0 48 16 9,026 8,411 Romain Barras 6 2.8 31 11 8,453 8,143 Bill Toomey 5 3.0 9 3 8,310 8,124 Attila Zsivóczky 5 2.7 24 9 8,554 8,222 Grigoriy Degtyaryov 5 2.3 14 6 8,698 8,328 Torsten Voss 5 2.3 23 10 8,680 8,359 Dean Macey 5 2.3 16 7 8,603 8,239 Kip Janvrin 5 2.2 20 9 8,345 8,101 Valeriy Kachanov 5 1.8 9 5 8,288 8,101
Max = most decathlons in a season
Mean = career mean of all scores >=8,000
Total = total scores >= 8,000
Seasons = number of seasons where score of >=8,000 achieved
PB = best career score
MeanScore = mean of all scores >=8,000
Tomorrow, I will try and look more specifically at times between decathlons.

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04262019 05:42 AMTo look at recovery requirements a bit closer, calculated the mean time between decathlons and the mean score for each year an athlete competed three or more times at 8k or better.
This specification resulted in data for 277 seasons by 133 athletes with a mean time between ranging from 17 to 94 days with half those seasons at 43 days or less.
The most notable of those with limited time between was Roman Sebrle in 1997 when he did 6 decathlons with an average of 24 days between (min 14, max 38) with resulting scores between 8,136 and 8,380 with a mean score of 8,284 and range of 244 points.
The most notable of those with a mean of 61 or more is Dmitriy Karpov in 2006 who had a mean of 66 days between his 4 decathlons (min 28, max 85) and scores between 8,293 and 8,438 (mean 8,382, range 145).
The distribution is in the following table:
Athletes MeanDaysBetween 26 130 87 3140 71 4150 72 5160 15 6170 6 71
That's quite a lot of variation across all the athletes,abd, for the two discussed above, short or long gaps didn't seem to impact on ability to perform consistently. However, this might not hold true if we look at all the athletes.

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04262019 06:01 AMAs discussed in the post above, the tales of Sebrle and Karpov show that short or long mean gaps doesn't seem to make any difference to them but this doesn't tell as about all the other athletes.
In order to get a better idea if there is any relation between the gap between competitions and performance level, I plotted all 277 seasons in the following image:
MDecMeanDaysBetweenVsMeanScore.JPG
From this plot, you can see that there is no obvious relationship between fortnightly, monthly or two monthly gaps between decathlons and the mean level of performance.
Of course, the might be a relation for certain subsets, so I looked at mean scores above and below 8,300 points and that looked the same as the above chart.
I also looked at the number of decathlons  3 vs 4 vs 5 or 6, and that made no difference.
I also looked at whether the minimum, maximum or mean gap influenced either the minimum or maximum score, and that made no difference either.
This analysis tells us that whatever recovery period is required for a decathlete, it is likely less than a fortnight.
Let me know if you are interested in any particular athletes or other analysis.

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04272019 08:33 AMFollowing olorin's request, I have now looked at the mean number of competitions by age for athletes.
When looking at he data it is clear that Euro and USA or USA trained athletes dominate the overall numbers, with very few other countries ever having athletes capable of producing multiple 8k+ performances in a year.
So I decideed to split the world into EUR and NonEUR because the USA athletes dominate the numbers anyway in the NonEUR classification, so this achieves pretty much the request.
Because an athlete could change ages during a competition year, I decided to assign the age achieved on the birthday in the calendar year to all competitions in that same year.
The analysis was also limited to years in which an athlete had at least three decathlons per year. This resulted in a total of 283 EUR athletes and 38 NonEUR producing 693 and 227 decathlons, respectively.
After all that work, there does not seem to be much difference between these geographical groupings at any age.
Please note that ages 2130 have at least three athletes per grouping but ages outside that range may have as little as 1 athlete per grouping. I can publish the table if anybody wants the detailed data.

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04272019 09:19 AMThanks El Toro.
Really surprised by the results  I was expecting a negative relation between the two especially for American athletes that are competing in the early 20' in the collegiate system.
My two cents for discussion. Few years ago, in the interview Ashton Eaton said that it takes him three months to recover from a full decathlon (two months for the heptathlon). When asked which event is harder the Pentathlon (5 events in a day) or Heptathlon (4+3) Kendell Williams answered that the Heptathlon by far because of he second day of consecutive competition take a huge toll on your body.

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04272019 09:34 AMI'm not sure about the opportunities for full decathlons in the collegiate system but, without checking, I would assume 8k+ scores aren't common for college athletes, let alone multiple 8k+.
We would probably need to have scores down to 7,600 to get enough younger athletes with multiple annual scores in order to fully compare USA to rest of the world.
Also the data set used here is also completed decathlons, so we don't know how many have been attempted. This might give a more interesting result. It might be possible to pull that data from IAAF but it would take forever.

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04272019 11:34 AMEaton was a different level though. Also  he did 5 in 2009 over 8k. Think it was 2009. Some of the talk about how long it takes to recover is posturing about how tough the dec is.
And almost all of the top guys would routinely do the dec and a bunch of individual and/or relay events in a 35 day period. And almost all of them would be training again within a week. The only long break would/should be at very end of season, where most would do 34 weeks of almost pure rest.
You can read Marra’a training approach (in detail) with a little work  and see that he doesn’t build in multiple month recoveries.