There is no date on a web design contract. If they don't provide, when can I break?

I signed a contract with a web development company to develop a website. Pay X for doing Y (about $5k). It's been over 2 months, and I've seen only marginal snapshots of what has been completed. There is no mention of dates in the contract. At what point can I say, "you have had enough time, I'm not paying you"? I imagine that after some period the contract becomes void for nonperformance, yet I need to give them a reasonable amount of time to develop something.