I usually only repot my plants in the spring or autmn. This is when plants go through a growth spurt and extend their root system. If you repot a plant in winter, the plant wont grow into the new space and the potting mix will just sit there and rot. This can be really bad for the plant down the track.
In terms of how often, I'd say as little as possible. I check under the pot to see if I can see any roots coming out of the drainage holes. This gives me a good indication of how cramped things are in there.
The plants above are all indoor plants. Most of them are over a year old and it's their first repot.
Re-potting is generally one of the last things I'd look to do if a plant was sick as it's very traumatic for the plant and in my experience is rarely the problem.