The brush should have a "wear line" - either a milled groove, or a painted line - about 1/4 of the "new" length from the top where the leads connect. How long it takes you to wear the brush down to that point depends on a lot of little things: how much current for how long, how much moisture in the air, what temperature at the brush running surface, whether the surface is a commutator or a collector or a shaft, what the brush material is, how "true" the running surface is, what speed you're running, how much contamination is in the immediate vicinity, etc.
First suggestion - read the operation and maintenance manual that came with the machine. Second suggestion - go talk to your local brush rep (for the brushes you're using). Third suggestion - go talk to the brush rep (that you want to switch to).
Converting energy to motion for more than half a century
