I would use a Persuade roll opposed by the "victims" Intelligence.
Most hypnotists that I have seen actually take some time putting their victims under, so say it takes 1-6 minutes giving the hypnotist the DM for extending the time from the "normal" 10-60 seconds.
In the situation where a psychologist is hypnotizing a patient, you could probably extend the time to 10-60 minutes.
Remember, a hypnotized person will not do anything that they would normally not do (won't kill themselves or anything like that), but there may be ways to "trick" them into doing something. If something like this comes up, I would give the victim another chance to break the control (another Intelligence check) with DMs appropriate to how out-of-character the action is.
If you don't like the idea of using Intelligence, you could use Endurance instead, since that characteristic is supposed to alsor represent willpower.
But, as posted above, it is also a burden on the player to make sure they follow the new requirements. Perhaps have the victim do whatever they are told until the PLAYER says "Wait, my character would never do that" and then give them a chance to roll out of it.
Hope that is what you are looking for.