I'd plan for 1 1/2 to 2 hours, but I'd be prepared to spend longer if you need to. It really depends on where you are in your decision-making process. I had no intention of buying the day I went and told them right up front, but I still spent about 1.5 hours.
The tour of the models is okay, but the real benefit is the Q & A time you have there. Be aware, however, that some of the guides are much more knowledgeable than others. I'd do a lot of research --- not only into buying direct from Disney, but also resale -- here on the DIS before you go. That way, you'll know what you don't know and will be able to ask the right questions. You'll also be able to laugh in their face when they tell you only SSR is for sale, or SSR is all that is available from Disney direct. So do your homework.
The guides are very nice, and there is no pressure -- but as Dean said if they have an incentive running for three days, that's for real, not just a sales ploy. That incentive may or may not be important to you.
You should also check here to see what incentives are out there. I'm not in the market, so I don't know, but there have been "Friends & Family" referral incentives in the past that could save big bucks. If one of those is currently available, get a referral from someone here and save some money.