How often do you change components?


A speaker review started off by stating the reviewer's reference system, and the recent changes he made to it. So how can he determine the sound of the new component in the chain? I know the audio upgrade bug bites hard, but if one continues to swap components, how do you keep your reference point? How ofter do you make changes to your system? How long do you live with th change before making another? Thanks
tbromgard