Idealy about every 5-7 runs.
After about 10 you start to damage the comm to the point where it requires a deeper cut to clean the comm up and this decreases the life the motor. Believe it or not but cutting the comm more often actualy increases the life of the motor and it makes sure performance is consistant.
You can get a cheap lathe for around $100 used. If you want new I recommend the Orion quick-set lathe. I really like how they have the screws to suppor the arm shaft so you do not need to shim the arm and use a pinion like the Cobra style lates. IF you have deep pockets the Fantom lathe is the best out there IMO. They run you about $400 but come with everything you need all setup in the box. All you need to do is charge the battery and solder up the switch and you are good to go. The Orion lathe will be a little less money and you can get a diamond cutting bit on ebay for uner $50 I think. Spend the money on the diamond bit as they will last forever and ensure you get a good cut everytime. Carbide bits wear out and you have to sharpen them or replace them every 20-25 cuts. I have been using the same diamond bit for 3 years now!
A lathe is a great investment if you are going to race a lot.