Every 3-4 weeks. Not just because I'm too lazy, but doing it every week wouldn't be far off doubling the amount of washes I do a month...
Having said that, I saw this thread, and hadn't done the sheets for about 3 weeks, so have just changed them, as well as tidied part of my room... God I ****ing hate mess!
Usually every other week, but that can change depending on if I've been on a night out and they feel a bit grubby or I've been ill and want to change them more/wait until after I feel better.
My mum makes me every week.
However it takes a long while for me to make the sheets in need of cleaning so when i get to uni itll probs be monthly.
Like i seriously dont release any form of bad smell for a long while, i barely wear deoderant cause my arms essentially never produce BO, i do wash daily but times where i forget no one (but me when i remember) notices. useful for uni i guess?