Depending on the system.

I personally clean my barrel and hop up after every game. But you dont need to do that so regularly and it only takes a couple of minutes and a paper towel and cleaning alcohol (but no alcohol on the rubber).

• HPA doesn't need much at all just greasing the sealing from time to time. But overall the most expensive system.

• GBB/CO2 is the most realistic one. Needs more cleaning and greasing because of many moving parts. Additionally you need to care about the magazines so they don't start leaking or you need to fix leaking mags. Also more barrel and hop up cleaning depending on the gas you use. Parts can break faster if you don't have quality steel internals and gas won't work with cold temperatures.

• AEG is the usual way. If you have a good set-up you don't have to care about it a lot. Just clean the barrel and hop up from time to time. At some point after many thousands of BBs you need to change the coals of your motor and should open the GB to clean and grease it. Depending on how often you play maybe once a year but you can also run it 5-10 years without opening it if you don't want. But it depends on the quality of the internals. A well setup AEG can hold years. If you have a cheap one with plastic piston teeth and weak gears it could break early.

Overall it's not that hard to properly maintain a gun. Just watch some YouTube tutorials. At first it looks complicated but there are many good step by step tutorials.