Not what your mom does when you visit (because you pee the bed frequently) but what YOU do in your own home.
why change your sheets when you can use febreze? seriously, i aim for once a week, and succeed about once every other week.
somewhere between a week and a month, depending on when i wash them and how many sets i have. both of which depend on a number of other variable factors. but generally somewhere in that range.
i suppose it depends on how many sets of sheets you have. if you have enough, you could change them every day and then just have a giant sheet washing day once a month.