what the depth is for is primarily water retention--it simply collects more rain and decreases how much/often you have to water it. also helps against severe freezes. i would suggest two feet, maybe substitute something cheaper than peat for the lower levels because the plant roots don't usually go down that far (i imagine really, really old plants eventually get that extensive a root system though). however, it has to be a material that will allow water to undergo capillary rise to the level of the plants' roots.
basically, deeper is better. it requires more expense and more initial work, but it isn't as likely to flood (don't know if that's a concern with precipitation in your area), retains more water longer (also doesn't evaporate as quickly), and is tough against drought/heat/freezes. IMO it's worth the peace of mind, and if you leave for a couple weeks or anything like that you can be assured your plants are alright.