We did this in December (booking through DCL) and had an absolute blast. As already noted, the full 3 to 3.5 hours includes start to finish, as we were not at Dolphinaris nearly that long. We had two couples and four kids -- two 10-year old girls, an 8-year old girl and a 5-year old boy. The 5-year old was the only one not able to do the belly ride on his own, but he was able to do it with his dad. If you do the full "swim," you kiss (and get kissed), hug, high-five, and swim/ride with your dolphin, as well as plenty of opportunities just to touch/pet.
Note that you cannot take your own camera. Naturally, they have a photograph and videographer with your group the whole time and are all too happy to sell you the pictures and video later! If there is an observer in your group with a really good telephoto lense, you might be able to get some decent shots (although that is even doubtful), but if everyone in your group is swimming, then be ready to do some bargaining with the guys in the photo room. With our two families, we left with some prints, all of our pictures on a CD, as well as a DVD of our whole experience...not particularly cheap, but quite a bit lower than the original "offer," so don't hesitate to bargain with them.
Our girls had the time of their lives with this (as did DW!...ok, me too!).
We opted to eat lunch there after our experience. It was actually pretty decent (how's that for an endorsement?!). If you have a lot of spare time, you might seek out some local favorites, but for convenience, I'd give it a vote!