I start all freshmen off with 10 minutes, the rest with 0. I then adjust based on the player's relative stupidity after I get the midterm grades. 

I put everyone at 5 and then dump then down to 0. I've had less then 5 guys flunk out on me in 50ish seasons.

My standard is 5 for Frosh and Sophomores.   Adjust up if the Underclassman is a doofus(2.5 or less), no lower than five for the Forsh until after midterms, then readjust accordingly.   Down to three for upperclassmen, with similar adjustments.  Call me overly cautious, but I usually end up with spare minutes on a player by the time they are a senior anyway and I don't like having to deal with a player being inelligible, after it bit me once with an important player.
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