Bubba's sheets are changed every week (more if she's sick/snotty like atm, every few days).
Older kids are changed fortnightly
Our sheets are weekly + doona cover as we don't use top sheets (just bottom sheet + doona)

Mattress protectors are changed everytime sheets are changed

I have 4 cot sets, and 2 sets per kids beds and 4 sets for our bed. I have a sheet addiction and love buying new ones all the time