A quick search says:
How do NFL players qualify for the pension?
Once a player has been on an active roster for three years (or on injured reserve) they become fully vested, which means they qualify for the full NFL pension.
How much money do NFL players receive from their pension plan?
The amount of pension money an NFL player receives depends on when they retire. For example, according to
Investopedia, if a player retired in the 1980s or 90s they could receive between $3000 to $5640 per month. NFL players who retire after 1998 receive $5,640 each month.