I have heard that if you can hold out for more than 24-hours they will let you go, but you have to sign away your first-born, if you aren't going to buy from them.
Actually, Marriott's timeshare presentations are probably the most laid-back in the industry. I have probably been to more than 10 of them. Everytime I get anywhere close to an MVCI property, I try and drag my wife to one just so I can get the 15K points or whatever they are offering.

In most cases, if I let them know upfront I am taking the tour only to get the points, they will let me off well less than the 90 minutes.
If you are at all interested in possibly purchasing a Marriott timeshare, I would strongly suggest you take some time and peruse the
www.tug2.net website. They are the experts on timeshares and there is a whole forum there devoted to Marriott.
Also, there are a number of threads on this forum concerning Marriott timeshares, tours, and tour packages. Do a search using the term "MVCI" and you should get enough info to keep you busy for awhile.