Growing up, we changed our bedding every Saturday. As a parent, I seem to only do it when "it's ready". Since my sons get baths before they go to bed and put on clean PJ's, and don't drool much, their bedding doesn't soil quickly. My 5 month old has pads underneath his head and bottom area that I wash every couple weeks or sooner, but I haven't changed the fitted sheet in a month or so.
My 4 year old has a tent bed made of Nylon, so I'll wash his blankets and pillow case every few weeks or sooner, but the tent bed goes about a month or two.
However, I'll change them immediately when there is an accident, they are sick, or something obvious is on it.
I work full-time and simply don't have the luxury to wash bedding sooner than is necessary.
I hope this helps.