Mine got herself of the nest every day even after day 18. Some days I thought maybe she'd been off a bit longer than I was happy with so I took her back to the nest. When I put her next to the nest and she saw the eggs she started clucking again and got back on the eggs, but I wondered if she was going to forget about them. She had been broody a couple of weeks before I gave her eggs so she had been sitting for about 5 weeks by the time they hatched.
5 weeks is a long time. Hens are different. When it was getting close to day 18/19 I would pour her feed in a pyramid shape--gently---the next day if it had not been touched and still in that shape----I knew she was into Lock--Down so to speak. They all would come off the nest in 2 or 3 days after not toching their feed with a nest of babies.