My tomato in a container needs a TON of water. I think mine's in a 5 gallon pot, but it could be 10. Anyway, I have to water it til it's soaking every day. I did notice yesterday that the dirt had eroded and some of the roots were exposed. I added more potting soil, and it didn't seem to be as wilty today.
When I was little, my parents would take a 1 gallon milk jug, poke a hole the bottom, and fill it with water. They'd leave that out next to the plant so it would gradually get watered over the course of an hour or two. However, with my (huge!) potted tomato, that just doesn't ever seem to be enough, so I just thoroughly soak it daily. The tomatoes we've gotten so far have been INCREDIBLE (and I'm not a tomato person)!