Francesca says: mattresses should be turned, ideally, at least once a month. Francesca admits to doing it only every six months, but it still extends the life and springiness of the mattress!
Here is a tip: Take a permanent marker and write numbers 1, 2, 3 and 4 at the various heads of the mattress. It makes it easy, when you turn it, to remember which side should now be under your head.
Here are instructions for turning a mattress.