It's a full day if you wish to experience all the shows. LMA, B&tB, and Indiana Jones are are more than a half hour long, and between seating and exiting there is three hours right there.
Add a sit-down meal, repeat rides on RnR and ToT, then GMR, Star Tours, Muppets, the Backlot Tour, the Little Mermaid Show, One Man's Dream, the Animation Tour...you can easily spend a whole day.
After you've been there a few times, it's easier to spend a half day if you skip a lot and just do the "highlights", but I still enjoy going to the shows and such so I usually spend several 1/2 days there.
NED