How often should I turn/rotate my memory foam mattress?

Memory foam mattresses should not be flipped at all - they only have memory foam on one side.

I think you should rotate your memory foam mattress every three months. For more information please visit: http://www.refreshmattress.com/product/mattress.html

SWEET DREAMS ... Rose's answer is correct only for combo mattresses, not pure memory foam ones. The combo mattresses only have memory foam on one side; the pure memory foam mattresses have it through and through and should be flipped whenever you notice an enduring imprint of your body after you've been out of the bed for hours. With a 2 inch thick memory foam this will occur about a quarter to halfway through the warranty period. The 2 inch memory foam mattresses usually are bonded to a non-memory foam (I call it a combo memory foam mattress or combo) part of about equal thickness (the kind Rose's answer alludes to). The combo can be turned instead of flipped; to turn it, move the part where you laid your head to where your feet are going to be so you will be sleeping with your head where your feet used to be. This helps extend the life of the mattress because we all seem to come to rest at a particular part of the mattress while we are sleeping and this movement will help juxtappose the worn part to a part not as worn. You can do likewise with the full or pure memory foam mattress; and you can flip it by turning it over so the side you sleep on will be the side that was down. You may want to change the orientation of the mattress cover (they usually have a crusty / hard side intended to protect the foam from whatever it is resting on - bed frame that is). Price around, pure high quality 8-inch thick memory foam mattresses are available for less than $200 (queen / king) and less than $120 (twin). Memory foam is second best and air mattresses are the very best sleep you'll ever get. Don't expect a cheap air mattress to last very long (it will last through it's warranty, usually not much longer, so price and buy knowing its lifespan). SWEET DREAMS ARE MADE OF THESE.