No single number, because it's a trade-off between risk and return. I've picked a risk profile (accurately so far, but only so far) and I'm on about 8% ANAR with about 18 months average age. I'd be happy with a 4% return on that, because I think that would be better than I could get with other investments with similar risk. If I needed more safety, I'd be happy with a lower return. If it were pure play money, I wouldn't be happy with any level of return, because it wouldn't be as exciting as betting on horses.
Edward