Would you go into it knowing you'll probably be killed before the end or would you have a reasonable chance of seeing retirement?
This excellent answer from the FAQ by u/Astrogator would likely help you as it is the same question but posed towards the auxiliaries. However, it discusses rates on the basis of battles and attrition across the whole of the army in general which may help answer your question:
Like the answer discusses, a Roman soldier's survival rate would have varied through periods, places, and specific legions. Is there any period and/or region you are particularly interested in?
There is a good and fairly short paper on this by Walter Scheidel, which is based on discharge information and broader population demographics. The little evidence that we have suggests that, in general, a the number of new recruits every years was a bit more than double the number of discharged veterans, so a little less than half of legionaries would reach end of service. In this figure, combat fatalities were not negligible but were also not determinant: by far the greatest killer of soldiers was disease and injury. Combat fatalities did matter a great deal in certain times and places, but by and large most of the time a legion was not fighting, so the hazards were unevenly distributed.