The EPA published a report in 1995 that describes the conditions under which phase separation can occur in oxygenated fuels. The primary oxygenates discussed in the report are ethanol and MTBE.
Phase separation occurs when the water content reaches the maximum amount that the gasoline blend can dissolve, any additional water will separate from the gasoline. According to the report, if the only source of water is from moisture in the air, phase separation is unlikely to occur due to the length of time required.
For example, at a constant temperature of 100 degrees F and relative humidity of 100%, it would take well over 200 days to saturate one gallon of gasoline in an open gasoline can (assuming the only source of water is water vapor from the air). Water absorption from the air is far slower at lower temperatures and humidities. (At a temperature of 70 degrees and relative humidity of 70%, it would take over two years to saturate one gallon of conventional gasoline in the same gasoline can.) Again, oxygenated gasolines can hold more water than conventional gasoline, and would therefore take much longer to saturate with water.
Source : Water Phase Separation in Oxygenated Gasoline
For those who would like a more visual reference, an individual in Hawaii put regular gasoline and E10 in glass jars and observed them over a four and a half month period. During the test period the temperature averaged 81 degrees and the humidity averaged 68% relative humidity. Pictures were taken periodically during the test and show that no phase separation occurred and visually the E10 appeared to be normal even though the regular gasoline had developed some haziness.
Source : Hawaii Gasoline Test