The train takes about 10 minutes.
We were there and back within a couple of hours - partly because it can't compare to being in Disney and partly because we found it to be nothing special.
We also visited the Aquarium which we toured around in a bout 30 minutes - compared to Aquariums back home in places like Plymouth and London it was tiny and not worth leaving DLRP for.
I'm sure other people will have had better experiences there but we should never really have left DLRP, and won't make that mistake again.