In what circumstances can you quit in the middle of pet sitting?

I would suggest, and I am sure Rover# would echo this that you let them know that you just can’t stay there again. You love their dogs. You’re willing to do drop in visits and walks, but you’re not comfortable staying there. You do not need to explain why you’re not comfortable. You don’t want them to go on the defensive because you’re criticizing their cleanliness or lack there of.

The other way this conversation could go is to say you love your dogs, but can't stay there. If you offer boarding, you could suggest that. If you don't, then you might ask them if they could book another sitter. Either with Rover re-booking someone else or maybe they have another option.