Part of having an Ethanol tune involves switching between the E30 tune and a straight 93 octane gas tune. In my case the 93 octane fuel is around 10% Ethanol from the pump and I have nearby (~30 minutes away) access to pump E85.
The Fuel-It Ethanol Content Analyzer has a companion application that allows some variables to be adjusted to help with filling up with each type of fuel to try and reach a desired mixture ratio.
Using the Fuel-It analyzer 0-5v output to a data logger the transition from E10 to E30, and from E30 back to E10, is recorded to evaluate how this transition occurs. The following charts show the transition for a few cases:
Sitting on a dyno with no load on the engine this transition took a long time.
It takes a few minutes for the new mixture to make it from the gas tank to the sensor, and then a couple minutes more for the final mixture value to be reached.
My procedure is to switch the ECU tune to the mixture I plan to use (93 or E30), then drive the car to get the new mixture through the fuel system. Since there is a mismatch between the tune and mixture for the first few minutes I take it easy. Driving more aggressively would probably cause the new mixture to reach the sensor more quickly as more fuel in the fuel lines is being consumed.
Since the mix transition process is imprecise, takes some time, and the nearest ethanol supply station is not around the corner, I keep a small stock of E85 on hand to help with getting the mixture to the right percentage.