There's an easy fix for this: simply ask up front (ideally in the 1st, no later than 2nd) conversation how many rounds (and how much net time total) they expect to need for a decision.
Be sure to very explicit about the upper bounds -- e.g. "4 rounds / 3 weeks at the outside".
And then - hold them to it. If at first they try to wiggle away - nudge them a bit. But if persist - just remind them of what was said at the beginning, give them a date X, and tell them that you'll be assuming lack of interest from their side, and pursuing other discussions from that point.
Or you can just tell them to go to hell, if that's your style. Either way, the point is: get the parameters up front - and stick to them.
