If we assume that it's a healthy, young, laying female, she might go off lay for as much as a week but I wouldn't expect it to be longer than that. When I brought home a Marans pullet that had already started laying, I got an egg the very next day. But, I have seen stress induce a pause for a few days before.Just assuming she is a lady and taking into account that she was apparently already laying before purchase, is around 20 weeks old, but has just moved house. How long should I wait before writing her off as a rooster?