well ya, your body type is fixed..if your genetically thin, then of course you can work hard and gain muscle to get stronger and bulkier, but your natural body type is still one of a skinny guy.
youll gradually return to your natural state, but it is pretty gradual. and also remember that even if you stopped working out, you still retain many health benefits that you gained while working out, such as stronger bone density and muscle memory.
but seriously, do you think your just gona work out for a few months and then quit? sorry, it doesnt work like that. dont be impatient; if your goal is to get big really quick so that you can find chicks, then muscle magazines and supplement companies will love you: youll foolishly buy into their expensive supplements that because steroid man in the picture says he uses it, and look at him..the magazines will love you cuz youll buy into their total bull**** routines.
like anything good that you cant simply pay for, theres not any easy route to reaching it.
persistence + patience = progress