The longer a negotiation takes, the more likely it is that it will not consummate as it allows the opportunity for other opportunities to come in to play that compete with what you are trying to get done.
This holds true for everything from hiring to closing sales opportunities to making acquisitions to seeking financing. The longer you let it drag on, the less likely it will happen.
Set a deadline. If it is a sale, base it on your closing cycle; if it is something outside your direct experience, take advice from experts about how long it should take; if neither of those work, just make an artificial deadline.
When that deadline approaches, let the other side know…and be prepared to walk away if your timetable is not met rather than waste further time and resources chasing something that isn’t going to happen.