As already stated, a player has 5 years to play 4.
This is known as the "Paul Blake rule." Named after the former quarterback for Texas St. University, back in the early 90's. Though he left school in his early 20's due to the unexpected death of his father, old NCAA rules kept his eligibility in tact. When Texas State discovered that Blake was eligible, they recruited him to return and play quarterback for the Armadillos while in his mid-30's. Ironically enough, that same Armadillo team was the first in NCAA history to play a female kicker.