May be no need to set a time limit if you are good at it. Most of the time when the climax is not happening it is because the lady is stopping every 30 seconds to ask if you are coming or have come. Hint if you have to ask it probably has not happened. Never had a problem coming in timely manner however when a lady provided quality service not stopping until the explosion.
To put another way good bng is no foreplay, no afterplay. But the quality of the actual service still has to be there. If you can provide that for a reduced rate one might consider going for it every paycheck as opposed to saving up and only seeing you a few times a year meaning more total roses.