Marie,
Welcome to the DVC family!
We are also DVC members, and we have opted to use the Tables in Wonderland program the majority of the time. I am far from being a math expert, but my husband is a financial analyst so I rely on him to solve problems like this. What he did was to to look at the reservations I'd made and see roughly what we'd be spending at each restaurant, and compare the discounted Tables in Wonderland price to the cost of the Disney Dining Plan. TiW always comes out the winner for us, so we've stuck with it.
If you'd like to try this for your own family, I highly recommend visiting
www.allears.net and clicking on 'dining', then 'menus'. There is a comprehensive list, including prices, for each eatery on property, and this is extremely helpful in planning a vacation. You can use it to get a rough idea of what your cost would be, and compare the savings that way to see which option works out the best for your party.
As for the number of days, you'd have to purchase the dining plan for your entire stay. The DDP is based on the number of nights, and if you choose that option you have to purchase it for everyone in your party for the entire length of stay.
Hope that helps, and have a wonderful vacation!