it's pretty hard to estimate how much they'll lose. There's a good chance they may not lose any points during the off-season in 5-6 categories. But stamina always appears to take a hit.
ie.. I had a guy with a 1 WE and high potential in Stamina. Begin his freshman year with a 62 stamina rating. Left 4 seasons later with a rating of 60. I think he gained a total of 13 points over the four seasons and lost a total of 15 points during the three off-seasons.