The frequency with which you clean your chrome exhaust pipes and tips depends entirely on how often you drive your vehicle. If you drive often, you will want to do a light washing on the chrome at least once a week. \
You should always clean chrome off exhaust pipes when your vehicle has been exposed to snow. Road salt can be particularly damaging to chrome, creating pits and damage to the pipes and tips. \
If you drive more rarely, clean chrome after every 50 miles or so. Use your judgment for how dirty chrome looks before you clean it but remember, it's always easier to clean when it is less dirty rather than dirtier. \