From top to bottom...
Assuming you pre-boost to L4 in the off-season...
If you at least wait until day 32 of season 1 (or after your last regular season game is played) you get 625 XP every game. That allows you hit L19 on day 40 when daily XP shuts off.
In season 2, if you did that, there is no benefit to be gained from waiting to boost as the next game XP plateau is L16-31, which you will get through either way. As long as you boost at or before L22 (boost into L25 or lower) you'll get max VXPs.
In season 3, which are the University leagues, as long as you've boosted every season so far, you start at L32. You can either boost to L35 right away to get the VXP increase (and play better), or eek out a few more XPs by waiting to boost until day 32.
However, at this point, as long as you boost every season, no matter when you choose to do it, you'll hit L79. The more often you wait to boost, the sooner your daily and game XP will shut off to prevent breaking the L79 hard cap.