For my GURPS DF / DFRPG game Felltower, I've generally been fairly aggressive on the restock. I check the real-world gap between games and make my rolls and decisions based on that time. The longer the gap, the more chances for new denizens and reinforcements to existing denizens.
I use my own guidelines, of course, because there isn't a lot about megadungeon play for GURPS in the rulebooks.
The past 6-8 months, though, I've been significantly less aggressive in my restock.
This is almost entirely due to real-world issues.
More time between sessions means an aggressive restock means a lot of time re-clearing places the group has already been. That can get old for the players and the GM alike. It doesn't mean I'm not restocking, just that I'm making sure it doesn't outpace the players' ability to clear and explore.
I also have less actual time to sit down and do it. More work and more study means less time to spend on game. So what's there needs to be sufficient. It largely is.
So I've been doing less rolling for restocks. I'm doing more manual decision-making. And I'm putting in less monsters to fill back up places left fallow.
But I'm still doing it.
And there are still plenty of wandering slimes and growing of spores, mold, and fungus.