The Mavics brushless motors do not require maintenance. Keep them free of dirt and debris and they will last years. The bearings on the motors are sealed and sealed bearings are different than shielded bearings which require oil every so often, or when they start to squeal. DJI bearings are very strong. The only time I have heard a Mavics or Phantoms motor squeal is after an impact and the motor was damaged.
As part of my preflight, I slowly turn my motors by hand with the props off. I feel for any resistance or a gritty feel which would cause me to look inside the motor for whats causing it. The motors are not available at local hobby shops, and if you find one that needs maintenance, and you havent crashed, it would be covered by your warranty if within 1 year of purchase.
The real maintenance items are your props. Some people continue to fly with loose or chipped props, but since they are what are keeping you in the air, you should inspect them before every flight. If they start getting loose on the motor, time for a new prop that will only set you back $5.00.