You should rotate and flip your mattress regularly so that you do not get irregular wear and tear. This is really important if you are large or heavy and especially so if you share the mattress with someone who invariably will be a different build to you.
You should flip your mattress at least once a season – so 4 times a year.
What would be even better is if you flip your mattress every time you change the sheets – every two weeks or so.
If you are flipping it super regularly, each time you change your sheets, you should flip it front to back one flip, and side to side the second flip. If you are not flipping it so regularly, you should focus on the side to side flip.