Here's one study:
"When assuming FGM, we estimate that 14% of new nonsynonymous mutations in humans are beneficial. The majority (98%) of these beneficial mutations have small selection coefficients, with
s < 0.0005 (
Fig. 4C). In
Drosophila, however, the model including positive selection had a similar fit to the data as the plain gamma DFE (
SI Appendix, Table S4), and only 1.5% of new mutations are beneficial (
Fig. 4D)."
They are also measuring how beneficial (selection coefficient) mutations are.
Source