Given that risk-averse investors demand more return for taking on much more risk while they invest, how much more return is suitable for, say, a share of common stock, than is suitable for a Treasury bill?

Even though we know that the risk-return relationship is positive, the question of much more return is appropriate for a given degree of risk is particularly difficult.  Unluckily, no one knows the answer for sure.  One famous model employed to calculate the required rate of return of an investment, given its degree of risk, is the Capital Asset Pricing Model that is abbreviated as CAPM.