Answer: The answer to your question is going to depend on a number of factors ranging from the type of soil in the bed to the ambient temperatures, wind conditions, and of course, rainfall. To some extent, the plants will tell you if they are over (or under) watered. You can also try to gauge the soil moisture by sticking your finger in a ways and feeling to if it is moist. A layer of mulch will help to maintain soil moisture as well as hold down weeds.
Ideally, you want to achieve a soil that is evenly moist yet well drained (meaning not soggy). The concrete most likely has a bit of slope to it, so I would expect the bed to drain adequately. In fact, it may dry out faster than soil below grade -- especially if you used a "soilless" mix to fill it.
The rule of thumb is to provide an inch of water a week either from the sky or the hose. When you water, if possible avoid wetting the foliage to help reduce any disease problems, and if that's not possible, try to water in the morning. Finally, a less frequent deep soaking is preferable over a daily light sprinkling. The optimum frequency for your particular situation though will depend on the conditions mentioned above, however. In the height of summer when the plants are large and producing heavily you will need to water more often than you would in a cooler moister period.
With a little experience with the raised bed you will become a pretty good judge of when to water, like so much of gardening we learn a lot by trial and error under our own individual garden's conditions. Good luck with your crop!
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