Inside walls, they might find tiny leaks or steam condensation, which might extend survival a bit. But still, not much beyond 3–5 days.
If you’re doing humane trapping, check every 12 hours. That’s the cut-off before dehydration causes serious suffering.
Seal food, water, and shelter points. Then trap every 24 hours. No water, no food, population control becomes more effective.
Fact check: Rats die quickly from dehydration. So yes, a trapped rat without water likely won’t survive more than a couple days, humanely speaking, check often!