This varies from one model to another. You can find this in your user’s manual. You have to refill the oil until you reach the marked level on the level indicator or the dipstick. Pouring too much oil could have severe consequences for your generating set: the oil could get carried around by the piston rings and be dropped into the combustion chamber. This process will produce black smoke and cause oil residue to drip into the exhaust system.
 
Oil has to be changed after 75-150 running hours or after a year, if the generating set has not been used on a regular basis.
Please note: this strongly depends on the type of engine!