Mmm a tricky question to answer in full! There are 2 parks:
Disneyland Park and Disney Studios along with Disney Village (like Downtown Disney but much smaller). You won't need as long there as at WDW, I'd say maybe the same amount of time as doing Disneyland California.
As far as when to gois concerned I'd look at the 'Really useful info' sticky and go to school holidays so hopefully you can avoid European school hols. Christmas is a wonderful time to go, as is Halloween. We're going at the end of November during term time so we'll get the Christmas experience without the crowds!
You could have a look at
www.dlrpmagic.com for a better look at the parks, rides etc or the official site
www.disneylandparis.com
Have a look at the stickies on this forum for info on hotels, how to get there etc.