I only change my kids beds when they lose that fresh smell/feeling. I don't have a regular set pattern for doing it and we are away at least one or two weekends a month. Sometimes it can be 3 weeks, even 4 between changes.
My mum strips her bed religously every week and i'm sure she'd be horrified if she thought i wasn't changing the kids as often.
i'd be interested how often other people change their kids beds. (kids are pre school btw.)