In general you don't really have to do a lot of maintenance to keep AEGs working. Except for some lubing after playing in the rain to keep the rust away and barrel cleaning. From what I know it is recomended to re-grease mechbox once a year. Some people don't even open them at all until something breaks inside. Gas guns require more maintenance, it's a good idea to keep them well lubed with silicon oil.
Stock AEG will last for years if you don't abuse it too much. Velocity will drop down over time but gearbox internals can work for a loooooong time with stock spring. Upgraded guns are different, once you start upgrades it will start causing problems...