We have an unused bedroom, that usually just keeps excess 'stuff' on a short term basis, but can quickly be habilable with a 10 minute tidy up. Over the eight years we have lived here, it has hosted a number of different visitors, for durations between one or two nights (usually grandma, or another overnight relative visit) and periods of three weeks to three months.
One of the challenges is how far from the core a place is. I am soundly in the suburbs. A few of the younger 'I want to go clubbing on Friday sorts' are not amused at the $45 cab ride home if they miss the last train/bus home option at 1am.