It varies depending on a number of factors. But, in terms of retaining moisture, adding mulch pretty quickly results in diminishing returns. In other words, four inches of mulch is typically only slightly better at preventing moisture loss than two inches, for example. On the other hand, there's a nearly linear relationship between how deep mulch is and how much moisture (from rain or overhead watering) is prevented from reaching the soil surface at all , especially if the water comes in relatively small amounts at any given time. It's easily possible to make your soil drier by adding too much mulch. That's why you commonly see recommendations to limit mulch to a depth of two inches or so. Under "typical" conditions, that's deep enough to limit moisture loss to evaporation, but not so deep that it interferes with infiltration. The challenge is figuring out which variables you have that are not "typical", and what you need to do to adjust for them.
