Biological membranes normally have different composition of lipids on the inside and outside (ref 1, ref 2). This is maintained both by how new lipids are added to membranes, and by specialized enzymes (flippases). Certain lipids being on the "wrong side" is an important signal (eg for apoptosis).
My question is: how fast do lipids from the inside and outside exchange spontaneously? In a patch of membrane with no enzymes, how long until half the "inside" lipids have flipped to the outside and vice versa through simple diffusion?
I did a search but couldn't find any specific paper that measures the spontaneous exchange rate. I'd be very interested in a definitive answer - could be just order of magnitude, but based on real measurements.
UPDATE This is important for figuring out the inside/outside composition of membranes in real scenarios when they're no longer being maintained in an asymmetric state: for example enveloped viruses, but also microparticles, dead cells, etc.