In Ancient Rome, auxiliaries would get land and citizenship after 25 years of service. How often would these soldiers fulfill those 25 years of service?

Hey, since no-one has answered this question yet, I'll give a copy here of my answer to a similar question that came up here before.

Additionally, to get two (absolutely common and understandable) misconceptions in your questions out of the way: Auxiliaries couldn't expect land grants after their service, same for money grants. Direct land grants in itself are more of a republican/early imperial thing, later on you'd simply buy some land from the legions territory at a preferential rate. The second misconception is that auxiliaries would always get citizenship, and that their length of service was fixed at 25 years. The granting of citizenship evolved out of the tradition to reward exceptional service by auxiliary units with grants of citizenship 'honoris causa' during republican times already, but only under the reign of emperor Claudius (41-54 A.D.) - in line with his general policy of integrating more peregrines into the imperial administration and military and extending the citizenship to more people and peoples - did this become a standard practice. It is also during this time that the first standardized military diplomas, documenting the grant and status of citizenship for an individual auxiliary veteran, begin to appear. But even then it was never a legally enforcable right, it was just something traditionally bestowed upon the veterans by grant of the emperor.

Furthermore, there was no fixed length of service for auxiliaries of 25 years for a long time. Honorary discharge after your 25 year term was not a formal legal right that you could enforce in court, but based on tradition. Furthermore, discharges only took place on specific dates, often two years apart, so that in reality you'd often serve longer, like 26 or close to 27 years. Some, especially those who made a career and became or wanted to become centurions, chose to stay longer, careers of 40 years are not uncommon for centurions, and there is no fixed end for their terms. Until Hadrian, auxiliaries generally served for 30 years before becoming veterans, so in general the length of completed service would on average (rouhgly) be between 27 and 35.

There is no hard data on this, so I'm afraid I can give you none. However, I can tell you a bit about what we know, how we know it and what that tells us about the probable answer.

I have written about the length of service in this thread, the short version is that service would often have been longer than 25 years, and usually between 27 and 35 years before Hadrian. It was not a hard rule that you got your citizenship after 25 years, it was a special honour customarily granted to you on your release by the emperor, whenever that happened to be.

For the pax romana, we can neglegt battle attrition, which has been estimated to be as high as 2,6% for Republican times (see Rosenstein, Rome at War, 2004) and 0,8% for the first and second century A.D. (Scheidel, Marriage, families, and survival in the Roman imperial army, 2005). However, the Illyrian and Germanic campaigns still led to battle losses - of course only for the involved units, so this is hard to generalize for all auxiliaries.

One easy method is to use life tables, which give rough, generalized ideas of how many of an age cohort would die within a year and of the life expectancy. While its true that ancient life expectancy at birth was skewed by high infant mortality, it's not like you could be certain to live to a ripe old age once you passed childhood - life expectancy at age 15 was ~50, which is 30 years lower than today. So we can expect a good amount of natural attrition during a soldiers length of service.

If we use such a model table (Frier's in this case) and calculate for 100 soldiers entering at age 20, we can expect about 56 of them to still be alive at age 45, or 44 to have died by natural causes. This drops to 48 survivors 5 years later. So even not counting any other influences, we already have little more than half alive, if everything runs normally and no war, insurrection, plague or border skirmish happens. Epigraphic evidence for the legions suggests that their rate of attrition in peacetime was roughly around 50%, so not significantly different from the general population (some fluctuations can be explained by promotions and transfer to elite units). Roman army barracks generally had amenities such as baths, latrines with water-flushing and washbasins or infirmaries that would help with preventing or curing disease. This would of course also be slightly different from region to region.

On your second point, since the Roman citizenship always remained a gift of the emperor (even to those who served their full term, so it could as well be withheld - and there was no legal right to enforce it - note also that this custom only started with Claudius!), units could be bestowed that honour prematurely (ante emerita stipendia; for an example the military diploma CIL XVI, 160), even on the battlefield - as was the custom before handing out citizenship to veterans became the norm. But to estimate this for the imperial period would be very hard. There are a number of surviving military diploma which contain this formula, mainly from Vespasian and Trajan, one unit which was distinguished in this way was the cohors I Brittanorum in the Dacian wars, but to say exactly what the demographic makeup of that unit was at that point of time and what your chances were is impossible to say. It happened, but rarely, so I would estimate a very low number. Individuals might also be granted the citizenship out of order, but that is statistically insignificant.

So if I had to give you a number, I'd say your chances were around 50%, if your unit didn't go on campaign during your service - if you were part of one of the auxiliary cohorts that joined Varus on campaign in Germany, or one of the auxiliary units in the Batavian insurrection, your chances would plummet rapidly.

And to add to that, look for the work of Scheidel, especially his exemplary case-study on Egypt if you want to delve deeper. Rosenstein is also reputedly great for republican times, though it's been years since I've read his book.

The reason I left battle attrition out is because it is highly dependant on the factors you listed:

period, location and branch of course

Giving an answer to that would probably be longer than a doctoral thesis. You'd have to compile so much data, account for so many factors and uncertainties, and in many cases simply wouldn't have enough data for a meaningful answer.

Hell, often we don't even know more about an auxiliary unit than its name, and sometimes even that is in doubt. And that's not even going into which campaigns they took part in, where they were stationed (and for how long!), from where they recruited, who their commanders where, and what losses they took in battle (and how accurate those numbers are). So, I'm sorry I can't give you a better answer, but that would be really too much to ask. Even arriving at such general estimates took decades of inquiries and fieldwork by multiple scholars. For some individual units, there is closer information available, but those have shown that those units (for example the Cohors XX Palmyrenorum Equitata Miliaria) had probably highly fluctuating personnel rosters, so one would have to know more about that to correlate that with military diploma and other epigraphic evidence that gives an idea about discharges in certain years, which is only available in fragments. I hope this isn't too frustrating an answer.

P.S.: I see that /u/Archis had much the same idea at the same time, so thanks for the mention as well!