Originally posted by English Pats Fan
Save them for as long as you want, I'd personally suggest saving them until your first Regular Season game as you don't really need to invest them any time before that as they won't have an impact due to it only being the offseason and preseason games 
if you are boosting on day 48 you do not want to do this as you want the ALG's to take place above the 1st cap
as soon as you can get to the next cap, spend the SP