Easter eggs don't keep very well. It will definitely be off by the time you start weaning. I'd keep it till Sunday but not much beyond then. I think they may have a "Once open eat within 3 days" label too.
After Easter I often find myself having to help DC's out by eating some( a lot) left over chocolate - we all know that once opened, it's on the turn and waste in this current economic climate should be a crime