Depends on the GPA of the kid in high school and class year. Jr's and Sr's rarely need time at all (the exception being juniors you recruit as JUCOs who come in with low 2.x GPA's)
Sophomores with HS GPA's over 3.0 I will often start with 0 minutes and adjust at the midterm if they're showing below a 3.0 GPA (usually I kick them up 5 minutes if they are). If they had a sub-3.0 in HS, I'll start them with 3-5 minutes, again adjusting as needed at the midterm grades come out.
Freshmen I typically give between 5 and 10 minutes to, even if the kid was a 4.0 in high school the game seems to be really fickle about grades the freshman year. Again, I'll bump 5 minutes at midterms for anyone showing a grade under 3.0 for the term.