It also depends on what type of brushes you have.
If you go to long, you will burn up the comm and this will cost you more than chaging brushes.
If it is just for bashing and not racing, you can buy some inexpensive 'low wear' brushes that will last a while.
You should still clean your motor out once and a while.
keep the brushes and springs and everything intact, to spray it out with motor cleaner and lube the bushings after.
This will give you the most life.