One of the Lair Actions of a lich does the following:
The lich rolls a d8 and regains a spell slot of that level or lower. If it has no spent spell slots of that level or lower, nothing happens.
A lich decided to attack the party, using some of its spell slots in the process, and then retreat to its lair to recover and await the party.
If the party manage to find the lich’s lair within 8 hours (i.e. before the lich can long rest and recover its spell slots that way), would the above Lair Action have (likely1) restored all of the lich’s spent spell slots (excluding the 9th level spell slot, obviously)?
Or is there some reason this wouldn’t work, such as the Lair Actions only being "active" when there are enemies of the lich actually in the lair (i.e. during combat)?
1 I say "likely" because there’s a dice roll involved, so technically it could not roll the numbers it needs to actually recover all its spell slots, but let’s discount that possibility as I imagine an hour or so of rolling will almost definitely roll the numbers that it needs eventually.