A party should get a full heal-up after approximately 4 regular battles, 3 hard battles, or 2 regular battles and 1 very hard battle.
For storytelling, try to sync full heal-ups with an appropriate event in the game world.
If the PCs find some special source of healing when they don’t deserve a full heal-up yet, allow them to get one or two recoveries back and to make some extra recharge rolls.
If the party is able to rest and decides to heal-up ahead of time, they suffer a campaign loss. The story moves along, but the situation in the campaign gets noticeably worse for the party (at the GM’s discretion).