In the G&G manual it tells you to spray some silicone oil into the hop-up, but it does not tell you how frequently to do it. I spray some oil around 2 to 3 thousand rounds, but everyone has their preferences. I try not too spray to much in there, and after I do load up 100 or 200 rounds and just rip through them to blow the oil through the hop-up and to get rid of any extra oil. After shooting the oil through the gun, get a clean piece of cloth or whatever you use and clean the excess oil out of the inside of the barrel, and thats it.
Most people using G&G feel confident in the gear box to last quite a while before taking it apart and greasing the gears and piston, I plan to do mine sometime after I shoot 10,000 rounds, but again some people may prefer to do this sooner, and some later, so you better decide what sounds good to you. Hope this helps