whether models can withdraw

if she gets tied up and the tickling starts, does she have to last a set amount of time or can she stop

I'm assuming you are talking about producer made films, so I'm going to answer along those lines.

A model better be able to stop the scene at any point they wish too. Any producer who doesn't respect the model's boundaries becomes a predator at that point. Especially if the model is tied up, a safe word or phrase needs to be established for before the play. This should be no different in private play.

The person in the most vulnerable position should have the ultimate power in all situations over the proceedings.

Rule of thumb is, Soles_Scream is right. I've shot a few videos where the model relinquished her safeword (the "No Safeword, No Mercy" series), but it was only with people I'd worked with before whom I'd already established a bond of trust.

When I was a producer, absolutely. All models were provided a safeword and they had the option to stop things at any time. To not respect their wishes after they said to end things would be criminal assault in my mind.