You should consider the decision to join a game a commitment to play it through. The GM will either be able to help you find a partner to team up with, will let you give it a shot and then help you find a replacement, or will ask you not to play unless you are willing to make a full commitment. Well, the game involves a certain amount of treachery. Going in you should know that. Good players learn from stabs.
Polite players stick it out.
Bad players quit. Worse that that, RUDE players quit. Be considerate to the six other players in a game and fight to the finish. Should this happen, let your GM know as soon as possible, not at the last minute. It is also good etiquette to try to find a replacement yourself so that your GM doesn't have to, either by finding somebody you know to substitute or by posting a polite message to Rec.