Last year (my first year using Square Foot Gardening), my cinder-block bed was only 6 inches deep, per Mel Bartholomew's "new and improved" advice, published in 2005.
Need I tell you what happened when temps hit 100-degrees plus last MAY???
Watering twice a day was nowhere near enough. The plants managed not to die, but only *just*...
This year, the bed is deeper, and during our 97-degree weather in April, the plants did very well with daily watering.
My 12-inch deep wooden bed is fine; once a day and no problem when it's hot. Once every other day to every third day is fine during the usual weird Bay Area weather.
Sunset Zone 17, USDA Zone 9