Well even the plastic gears should last some time if you're not overvolting and running them hard (big wheels, powerful batteries, harsh driving, etc) but they will fail eventually. The resulting plastic mess will go all around your gearbox and get stuck in the grease, and the only way to ensure they run smoothly from then is to strip them completely (toothbrush + warm soapy water, in our experience). Any plastic bits left inside and it'll never run smoothly.

I'd save yourself the whole hassle of that and just buy some metal gears from Gimson. It's worth the money in time spent having to mess with the gearboxes once the nylon gears fail. Plus there's a chance that them failing will cause the gearbox to lock up, and you could burn a motor out... point is it starts costing money quickly!