I think school prestige in D1 has been 4 seasons as long as I remember.
D2 & D3 are different as they have a rolling prestige grade based on prior seasons, combined with the current season. But as mentioned earlier this is weighted to the most recent seasons having more value. The most recent season could be somewhere near 50% of the new prestige.
Prior seasons could be viewed as lineal; 25%, 12.5%, 6.25%, 3.12%, 1.56%, 0.78%....