Change of Ages

Riddle me this riddlers. How long do you hang on to a book agent?

What I mean is, you have spent years writing and perfecting your book. Then you spend another several years submitting to agencies when lightening strikes and you're finally accepted. Yet three years later your book doesn't appear to have gone anywhere and your contract is up.

When do you call it quits and look for a new one? And if you do find an agent who successfully gets you accepted into a publishing house are you still obligated to the original agent in any way?

Just some thoughts floating around in my head.