You can't neglect soil type with this, as your soil type basically drives the answer to your question, and the answer will vary by region because of it.
For example, 2:1 is the usual limit for stuff in central to north Georgia, which has clayey soils, but 3:1 is often required if you're going to seed it instead of sod it. 4:1 is generally the limit down in Florida, where land is flat and soils are sandy. Sometimes regulatory agencies in hillier areas with clay soils will allow you to go 1.5:1 if you sod it, and you *stake* the sod.
Your local land development code usually addresses this issue, and increasingly those are available online. Fish around Google, or possibly Municode. Also don't be shy about talking to your local reviewing agency. Chances are you'll have to get a permit anyway, especially if you're including any drainage works in your plan, which it may very well need.
Basically, you're looking at a berm that's anywhere from 27 to 72 feet wide, depending on local regs. I don't know what your land plan is like, but if losing that real estate gives your developer heart burn, which I find it often does, he'll change his plan fairly quickly from "earthen berm" to "privacy fence with vegetation."
If you get to the point where you are indeed going to do a berm of some height, pay a lot of attention to the drainage so you don't create any birdbaths on either side of it. You may need breaks in the berm, or culverts of some kind. The first time you flood your neighbors out, that berm gets a lot more expensive.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
