I'm with Katherine: Usually there's some reason why people give the "I need to think about it" answer. Either they know and don't want to say, they don't have enough information, or they just haven't done the hard work of deciding (balancing +s and -s, making a commitment), etc.
That said, sometimes people do need time to let it percolate. In that case, set a firm deadline, one that's not too far in the future (e.g., a few days, two weeks, etc.) and go ahead and make an appointment *now* to reconvene and decide. Also, agree that there's only one extension.