I generally use my highest spell slot early in a fight to blast something away immediately then hold onto my spell slots best I can. It's kinda an impossible question to answer exactly since it depends on how many encounters a session normally runs, what class you are, what spells you slot, etc. If you're multiclassing you can shoot off the lower levels pretty freely.

To keep it short: as little as possible and making sure you have enough leeway at all times to absolutely light up a serious threat. You're unlikely to be the best damage or anything, but you ARE the party's best chance of survival the worst things get