The largest Ganesh statues, on display to the public, are usually immersed on Anant Chaturdasi -- 11 days after the first day of the Ganesh festival. However, many people who keep a Ganesh in their homes choose to perform the immersion much before this. There are no hard and fast rules as to when it's done. Rather it's up to personal preference. However, the immersions do follow a schedule and are only performed on selected days.