First, let's clarify what is meant by rotating the mattress. Rotating is when the mattress is turned around 180 degrees. The result is what used to be the head of the mattress is now the foot of it. Note that rotating does not involve flipping, so after rotating you would still be sleeping on the same surface of the mattress.
For optimal care, you should rotate your mattress fortnightly in the first four months, and then monthly thereafter. This ensures even wear of the comfort layers. If you and your partner are considerably different sizes or shapes rotation more often may be required.