I can only think of a couple instances where the timing itself would matter in terms of how much credit you get for the visits:
- If the recruit has a Wants to Play preference, you would want to promise him minutes at the same time or before you spend the visit money to maximize the impact of the visits
- If you're expecting a prestige bump after the season (and the recruit is a Late signing) your visits will have more impact after the prestige bump. So it could be beneficial to wait from that standpoint. Conversely, if you're expecting a prestige drop, probably better off spending it before your prestige drops.
Otherwise, purely from a "maximizing the impact of the visits" standpoint, I don't think there are other factors that would matter. Spending it all early, spread throughout, or late would have the same impact.
Now - from a strategic standpoint - there are things you would want to consider. Should you spend early to try to scare off competition? Would you rather wait and see how the broader recruiting landscape plays out before using your funds? That is very much a case-by-case decision.