How fast do lipids on the inside and outside of a lipid bilayer exchange?
A cursory search seems to show at least a few papers using sum-frequency vibrational spectroscopy to measure rates of flip-flopping/transverse diffusion in lipid bilayers.
This paper in particular (&quot;1,2-Diacyl-Phosphatidylcholine Flip-Flop Measured Directly by Sum-Frequency Vibrational Spectroscopy&quot;) cites an activation barrier of 79 kJ/mol which corresponds to a rate on the order of 10^-4/s.
There is also this review which seems to have more information on the topic: http://dx.doi.org/10.1021/acs.accounts.6b00435.