Check brakes light on dash on 2008 Mercedes-Benz C300

how long do I have?

3 answers
There is a recent recall for back lights switches. Go to the dealer , free fix
Usually registers when pads are getting worn out. Maybe 5000 miles tops, if you push that long the pads could start grinding on the rotors. Probably less depending on how and where you drive.
approx 1500 miles but don't wait that depends on driving habits