5% is the number that came to mind for me. Though it's likely because it's something I've read somewhere in the past. I doubt I'm clever enough to come up with that myself
It does make sense for the return to be at least high enough to account for inflation, which is less than 3% currently if my Google search is to be believed.
So all in you're looking for a return in the range of 7% to 14%. I guess the gold standard is to do better than the market, which will depend on the time frame you're looking at. I think historically that's going to be around 12%, or doubling your money about every 7 years. I could certainly live with that!