Legionaries had an obligatory 25 years of military service. The first 20 were spent as regulars, and the last 5 as reservists.

That being said, most of the legionaries didn't campaing the entire time they were in the legions. Most of their life was spent building infraestructure, clearing fields for crops, training and maintaining their equipment.

In Augustus' era, the average age of conscripts was around the twenties, even though there is proof of younger men being enlisted. In that sense, most legionaries were obligated to serve until their late 40s or even 50s.

The problem with a retired legionary's life is that their only gain after the 25 years was a Roman Military Diploma , which gave the legionary the right to receive pensions from the Aerarium Militare. Yet this pension usually was not enough to cover the financial needs of a retired legionary and his family. For that reason, most men pursued the military life in hopes to gain promotions and essentially make a living.

But if you are still wondering how many men made it to the obligatory 25, it's a rather high percentage of them. An estimate stated in A Companion to the Roman Army (Oxford: Blackwell, 2007) indicated that a simple attrition by natural disease chart would indicate that under natural, non-campaigning situations the casualties were of one third after the 25 ages of service, meaning that from 100 initial men, only 60-70 would endure naturally. Battles and campaigns would decrease the survival percentage a lot, but there is no certain way to estimate it. If you want to add a number to it, you may say that only 30 out of 100 made it through the service years.

Battles were not the biggest problem of the time, it was diseases. Most casualties were not caused because of lethal battle wounds, but by the resulting diseases those non-lethal wounds caused because of the highly unsanitary conditions of military camps and the early medicine of the time.

Edit: typo