Bottom line - here are the ranges:
Historic evidence: 4.3% (but very noisy)
Implied (from stock prices): About 5%
Consider this:- if the premium is about 4%, and the long term bond yield is about 4.5%, then you are looking at an 8.5% return on stocks. 8.5% is a far more modest return assumption than the 10-12% number you often hear as the "long run return on stocks".
Pretty much essential reading for all finance students.