An electric motor can be rebuilt as many times as you want to rebuild it as long as it's actually worth the cost to continue to rebuild the electric motor.
As long as the stator and other components and the motor housing are all in good condition there's really no limit to on how many times an electric motor can be rebuilt.
Although wear and tear on the electric motors core and the cost of repairs and the availability of of any spare parts can eventually make it less practical and less worthwhile to rebuild the electric motor.
If it's a cheaper and smaller electric motor it is often cheaper to buy a new electric motor instead of rebuilding the old one.
Although for larger and more industrial sized electric motors that are more expensive it can be worth it to rebuild the electric motor several times.
To maintain an electric motor and extend the life of an electric motor you should make sure you don't overload the electric motor, allow it to rest, ensure you oil and lubricate the electric motor often with the correct oil or grease and keep the electric motor clean.
Also ensure that the electric motor is getting the correct voltage and no under voltage or over voltage which can lead to overheating of the motor.
If possible regularly inspect the bearings and also clean the electric motor and keep dust away from the bearings and windings as much as possible.
Also reduce any vibrations and replace brushes if the motor has brushes and tighten any electrical connections.
If maintained properly and not overheated or overloaded an electric motor can last around 10 years or longer.
When oiling an electric motor you put between 2 to 4 drops of oil into the electric motor for each bearing, depending on the size of the electric motor.
Generally you can just put the oil into the electric motor a drop or two at a time until the oil reaches the top of the oil port.
Avoid over oiling the electric motor though as it can cause damage.
Electric motors require oil if the electric motor has oil ports which are located on each end of the motor housing where the motor shaft exits the motor.
Some electric motors have sealed bearings and don't require any oil and if there's no oil ports or grease ports the electric motor does not need any oil or grease.
But if the electric motor does have oil ports or grease ports they need oil or grease to keep the bearings lubricated.
The kind of oil that you use in a small electric motor is often 3 in 1 motor oil or SAE 20 motor oil.
The best oil for electric motors including small electric motors is 3 in 1 motor oil or SAE 20 motor oil.
3 in 1 motor oil or 3 in 1 oil in general is best for electric motors for lubrication.
3 in 1 motor oil is the best for electric motor bearing lubrication.
SAE 20 motor oil can also work in electric motors for lubrication.
WD-40 is OK for electric motors when cleaning and freeing up any stuck components although WD-40 shouldn't be used as a lubricant for the bearings as WD-40 is not an actual lubricant but is instead mainly a cleaner and penetrating solvent.
3 in 1 motor oil is different than the regular 3 in 1 oil.
Some larger electric motors may require grease or a thicker oil depending on the size of the electric motor although most household sized electric motors can use 3 in 1 motor oil or SAE 20 motor oil.
10W 20 motor oil or other light oils can also work for small electric motors.
And for electric motors that need grease, the grease consistency preferred for electric motors is normally NLGI 2 or 3, with a base oil viscosity of 100-150 cSt @ 40°C or 104 F.