I was trying to figure this out myself. It does depend on a few things: how hot it is and whether your garden is in containers/raised bed or directly in the ground. The general consensus seemed to be once a week for a garden directly in the ground. You are supposed to water really deeply when you do water too, to encourage root growth. I live in the Central Valley where it's hot as heck and I have raised beds/containers and I still don't have to water twice a day. You don't need to keep the soil wet all the time, that may even eventually rot your plants or cause mold. It's ok if the top of the soil is dry, but if you stick your finger in, it should be moistish down lower, and should stick to your finger a bit like an undone cake. The top of the soil being dry will actually act like mulch and keep the rest of it moist. Little waterings twice a day is supposed to be the worst way to water (so I was told). It encorages evaporation and may hinder root growth.
I thought these links were helpful:
http://www.vegetablegardeningguru.co...intenance.htmlhttp://pubs.caes.uga.edu/caespubs/ho...water-veg.html
For me, I water my raised bed in the morning, deeply, every 3-4 days depending on the temps. If we're in a heat spell and it's 100+ everyday, that's when I'd go with every 3rd day but otherwise, every 4 or even 5 days seems to be ok. I used to water every other day at least but I realized later I wasn't soaking it enough. My container stuff gets watered every 1-2 days, it dries out a lot faster.
HTH