I've read, at least, the substrate should be changed every week. Is that about right? Or is it more or less depending on how dirty it gets? Obviously I wouldn't want to wait till it's filthy to clean, but will occasional spot cleanings suffice and possibly changing substrate every 2-3 weeks and scrubbing out the cage? He's little and the only one in there and so far hasn't made much of a mess. Although I did already clean out his cage once just to be on the safe side.