I'd do four so you have one for each square. Start with an hour and see how it goes. It will leak out of the bed if you go too long so check often the first couple of waterings. Frequency depends on if it's mulched and how densely it's planted. I'd start with every other day if you mulch........may be able to do it every two days. Our community beds are compost with some sandy loam so they may hold water better....or not!