You should wash sheets as often as once per week.
To keep your bed sheets clean and yourself healthy for hygiene reasons it's recommended that you wash your sheets at least once per week.
I keep several sets of sheets for my bed as well as the kids beds and wash all sheets at least once per week to keep them clean.
Sometimes I'll change the sheets every 4 days especially if I'm sweating more often or my kids are sweating more often.
It's always nice to have fresh clean sheets often as having fresh clean sheets keeps the bed clean and helps me sleep much better.
But you can get away with washing your sheets every few weeks if you want too but it's not recommended to go any longer than that.
Of course hotels and motels change sheets everyday or when the guest checks out which they have to do for cleaning purposes.
But you don't have to wash your sheets that often.
Still you could replace your sheets with already washed ones everyday or every few days if you want too and then wash all the dirty sheets on one day.