How do you handle that? Do the PCs have to leave the dungeon and go somewhere to heal up a few weeks before going back?
I haven't run a straightforward dunegon crawl in a while, but yeah that's how it'd have to be taken care of. I wouldn't make them have to go back to a city or town if they had adequate shelter and food in the wilderness outside the dungeon though. They just need to be somewhere safe enough to lay around and do nothing.
Now if the rolls go alright it's not going to take very long for someone to heal up to full anyway (1d3 HP back per day if they're above half health, double that if a Medicine roll works out, most characters have like 4-6 HP per level on average, etc.), just a few days is usually plenty. It's usually a question of "do we have time to go heal?" more than anything else.
Things get really interesting when someone decides to press on despite having only 1 or 2 HP left too!