One lady on this board used to offer free outside social time with bookings of 2 hours or longer of private time. But she took a lambasting by many of the other ladies on here.
I think the unlimited time, in an "Up to xyz hours" is a good business model for some, but it would mean that the minimum price would be much higher to average out the potential length of the meeting. After all, a lady couldn't make plans, personal or professional, for any of the 'possible' hours she may be engaged. You are basically asking somebody to put their life on hold for you, because you 'might' want it.
Most gentlemen looking for that kind of scenario will simply book a much longer liaison (which is usually quite reduced from an hourly rate) and let the lady know that they may or may not need the entire time, but will pay for the entire block so that they can relax and take it easy. That is why longer engagements exist. Not to run a marathon from minute one to the end. But to allow things to play out in a more natural and less accelerated fashion.
With that said, when we are enjoying ourselves, and there is nothing pressing to attend to on the back end of a liaison, trysts will often run longer. For me, this happens predominantly with my 4 hour specialty packages. I like taking my time, so it's not unusual for them to run into 5 or 6 hours......or for a 4 hour PSE session to extend to 6-8 hours
However, I think it is also important to point out that if you do have a special lady who is relaxed with timing for you that you don't come to expect that as the norm. Sometimes it is a one time experience because chemistry and life just aligned properly. Other times it may become so common that you feel it is the new norm. However, you are still just booking the time frame you agree upon, so enjoy and appreciate the extra time rather than coming to believe it is expected.