Not long.
If youre going to do your wheels then by all means spare some C5 for the exhaust.
Personally I wouldnt bother atall, Id simply buy some "light" metal polish and polish it up every so often.
I maintain a new F10 M5 and I treated the tips to C5 when I did the initial detail. Lasted about 2 weeks. Now I just use brilliant metal polish to tart up the tips.
Same with my GT exhausts, tried using C5, lasted about 4 weeks on mine. Now I do the same with the M5, just polish with a light metal polish every so often to remove the crud.