Expected rates of return relative to safety over next 15-20 years

Don't know if this has been addressed somewhere previously in this forum, but I'd like to know a basic framework for various investment types relative to the levels of risk.
For example:

Zero risk = x type of investment = x % return

Low risk (safe unless total collapse of economy) = x investment = x %

Moderate risk (fairly safe unless cyclical collapse of market, but likely to return to prior level after a few years) = x investment = x %

High risk = etc.

Also, if one were to invest a large sum of money, what percent would you allocate to each risk level, and what would your attainable overall goal be for % return on long term investment?