[zdragnar]

As a guest bedroom? A few times a year, when distant friends and relatives top by. As spare storage for seasonal clothes, decorations and so forth? Probably 1 out of every 4 days. As a hobby room for things that don't work out well in the office? Quite a few more days. As a sleeping room for pets (ferrets)? Every day.

Edit: To be used as a guest bedroom, we tend to reorganize a bit (such as moving the pets to the master bedroom) but primarily we consider it the "guest bedroom". It sounds a bit nicer than "arbitrary spare room".