I keep two or three fake eggs in each nest box - for weeks and weeks. I recently combined two flocks in a new coop. The fake eggs have been in the nest boxes since I moved them into the new coop 3 weeks ago - and I plan to keep them there for a few more weeks because in addition to the hens already laying, I have some pullets that are just a few weeks away from laying. So I'll keep them in there until then. I also hoping the fake eggs will encourage one or more to go broody.