Think it will really depend on the child. I've seen some kids adapt and integrate really easily (i.e. a couple of days), and others take weeks/months.

If the boys are struggling to settle in, are you able to fast-track the "getting to know you" process with some after school play dates? Or could you encourage them to join a special interest group/sport team within the school?

Some kids will also need to find closure for the school they've just left before they can embrace the new one.

Hope you find the answers you're looking for.