The trick is to always let them be successful at reaching the goal, slow down as much as needed, say "two and half, two and three quarters......", they should never find out that I have absolutely nothing planned for the case when it finally fails.
And yes, 10 more times down the slide can easily turn into 9 times in a row and holding on to that 10th time for as long as possible.
Counting slides is something I don't intend to do, because I am not actually interested in micromanaging those 5 minutes.
Both you and the OP are right. Phrasing like this example though should help. This way it's not you restricting the kid, and there's no hard requirement.
If they try to drag it out, you tell the kid that they had the time; it was their choice to take longer than usual to go down the slide those last few times.
That explaination also helps them realize that they can choose to either make the best of a situation or the worst of a situation, but the situation will happen regardless (the time limit).