The first time I went to DLP I spent about 7 hours in
Disneyland, and that was enough to take all the major rides (BTM, Space Mountain, Star Tours, Indiana Jones, POTC), but not enough to take in everything (and I didn't even enter WDS that visit). It took me a second visit to see more of Disneyland and WDS, and a third visit before I found time to see Animagique and Cinemagique (which are both pretty good).
So it depends on how much you want to see and if you want to make time to see the shows/parades as well as to go on all the rides!
Would completely agree with previous posters about EMH - the queue times are really short during EMH (when I went earlier this month, I walked on to Peter Pan and Buzz Lightyear during EMH, both of which had >60 minute queues later in the day). An alternative to staying on-site would be to get an AP - Dream and I think the 20th anniversary Fantasy give passholders EMH as well (in addition to other benefits like merchandise/food discounts) - it could work out more cost effective to get an AP and stay off-site than to stay on-site depending on offers and season (but you'd have to check and work out prices for this!).
Train wise - an alternative to the direct London-Disneyland train is to take a Eurostar from London-Lille, then a TGV from Lille-Disneyland (The Eurostar website allows bookings for this 3 months in advance). It takes longer than the direct train due to the transit at Lille (which can be 30 minutes to 1.5 hours - though 1.5 hours isn't too bad as there is a shopping centre right next to the station), but there are much more timings available and I've found it easier to get a £69 return using this route than with the direct train.