Early in the recruiting period, it's rare a kid is going to consider you for anything less than 8-10 evals. So anything less than that is probably a waste. If you throw 10 at him and he's still not considering you, then that's where it becomes a bit trickier to know how much to keep throwing at him each cycle to pull him down. Some subjective factors include how bad you want him, how bad you want to get on his list first, your general risk tolerance, how early it is in recruiting, how good a player he is, cash available, etc.