At some point today my broody stretched her legs, and the proceded to get back on the wrong nest. When I got home from work she was happily trying to hatch absolutely nothing. The nest box next door had 8 cold eggs.
Unknown how long they were uncovered, but it could be up to 12 hours. Eggs are ambient temperature, so longer than 1 hour I would guess.
When I put her back in the right nest, she arranged the eggs and sat back down on them again like nothing had happened.