I remember a few years ago the saving interests in Australia was around 8% plus. That was risk-free rate, if you take a bit more risk on other financial products, 10% might be achievable. Not feasible for 20 years though.
This link says current interest rates are 3.5%. [1]
[1] http://www.infochoice.com.au/banking/savings-account/high-in...
Long term US stock market returns seem to be something like 10% but those definitely have significant risks involved, even for index funds[2].
[2] http://observationsandnotes.blogspot.com/2009/03/average-ann...