1. Brushes have no 'set' schedule for replacement. There are too many factors that can affect wear. The only thing you will have to go by is the brush length when doing your PM.
2. PM should consist of the following:
Spark plug check/cleaning/replacement as necessary. Gap should be .025"
Air filter check/replacement (never clean one) as necessary.
Blow off entire genset with compressed air - never use water to clean.
Check brushes and slip rings.
To do so: Remove the air cleaner. Directly behind the air cleaner is the end bell of the generator. In the middle of the end bell is a rectangular plastic cover. Use a flat tip screwdriver to pry it out and expose the brush block.
In the center of the brush block are 2 round holes with a single bare wire coming out of them. These are the brush leads. Use a small diameter probe to stick down in these holes until it hits the brush. Mark the probe where it is flush with the brush block. Remove the probe and measure from the mark to the tip of the probe. If this measures 1" or more, the brushes are worn and must be replaced.
Leaving all wires hooked up, remove the 2 screws securing the brush block by turning each screw about 4 turns at a time. The brush block must come out straight, as it is made of ceramic and is very fragile. Be very careful to not drop the screws. It can be harder to get them out than to get a lollipop from a 2 year old. Use the wires to hold the brush block and guide it out.
Examine the brushes. They should not have any chips or cracks and should be worn in a smooth arc pattern. They should slide freely in the slots in the brush block. Clean the brush block with electrical contact cleaner.
Now examine the slip rings. They are the 2 bands that the brushes ride on. They are made of copper and should be as shiny as a new penny. They should be smooth and not have any scratches or pits. If they are dirty, clean them by attaching a piece of scotch brite pad to the end of a small stick and holding it to the brushes. Remove the spark plug wires and press start to crank the set and make the slip rings turn. WATCH YOUR FINGERS!
Adjust valves per the schedule in your owner's manual. The intake valves should be set to .005", and the exhaust to .013"
I have attached a copy of the Owner's Manual maintenance schedule for you, if you do not have it.
Click here to download it.