To answer your question the eggs won’t start developing until she is in them almost consistently for 24 hours. After that they will start to develop. Until then she may leave the nest for a great many hours and adding additional eggs. Once she hits whatever number she is going to lay she will sit and stay with it.
but if she thinks that nesting area is not very safe she could leave it and move to another spot and start laying again. Be very careful being close because they can still smell you have been there even if they haven’t seen you there.