Hi Nick,
As you read through the FAQ area you should quickly work out that the "how much, how long" is almost impossible to answer. The points system that Disney has put into place offers great flexibility. I'll try to explain it VERY briefly.
There are 4 different unit sizes they are studio (4 people) , one bedroom (4 people), 2 bedroom (8 people) and 3 bedroom (12 people). Basically the bigger the room, the more points.
There are 5 (I think) different "seasons" obviously the more popular a "season" the more points a unit cost.
Finally what can make a huge difference to the economics of the deal. Sunday to Thursday points are much cheaper than Friday and Saturday, if you're able to avoid or minimise your weekend points usage it can make a huge difference. A decent rule of thumb is that weekends costs about 2.5 times as much as a weekday.
Disney is currently selling points for Saratoga Springs resort and Spa, these have a "shelf life" of 50 years, the other
DVC resorts (only available through resale) revert back to Disney in 2042.
Part of the beauty, IMHO, of the points system and the different unit sizes is that as your family's size and needs change you can adjust your bookings to fit.
Say at the moment you have one kid aged 4. You'll probably find a studio for 2.5 weeks, would be more than adiquate for your needs.
10 years time you have 2 kids age 14 and 8, you might prefer to go with the one bedroom unit for 9 days (separate bedroom for mom and dad, kids get the pull out sofa in the living/dining room to share , or buy an airmatress and let them camp out)
15 years time and oldest kid wants to bring a friend/partner you can get a 2 bedroom 5 days and rent a house/condo for the rest of the time.
20 years time kid #1 doesn't come away with you anymore so you're back to a one bed.
25 years time neither come with you, you and DW enjoy a long stay in a studio. 30 years time you take child #1, their partner and your grandkids away in a 2 bedroom.
35 years time save up a couple of years points and take both of your childrens families and stay in a 3 bedroom unit all together, or maybe alternate one family group one year, one the next, then the next a holiday with just 2 of you.
I don't think many Brits use DVC as their total accommodation requirements for every year. Many people use it as a quality treat mixed in with other choices. That way you can make the best use of the cheaper weekday costs.