The oil shouldn't ever drop out of solution (stay mixed) unless the methanol soaks up too much moisture. Someone in the Aircraft engine forum did some tests mixing various amounts of water to the fuel and testing it in an engine. Performance didnt change much, but once you get to about 4% water in the fuel, the oil will drop out of solution. Reshaking will mix it in for a short time but one could consider it spoiled at that point. If you have fuel thats sat for a long time, check the bottom of the jug - if there's a "bubble" of liquid on the very bottom of the jug separate from the liquid on "top", its probably moisture laden and the oil dropped out.

At say 1/2 gallon, 4% water would be a little over 2.5oz. Thats a lot of water - probably more than what it could absorb sitting open to the air for a whole day. Keep it tightly capped and out of direct light and it'll last a long time.

On fuel consumption, My Aircraft engines eat more fuel than my car engines but they're also twice the displacement. A gallon lasts me all summer, but now that I mix my own fuel I'll probably use more this summer. I ran 2-3qts in my aircraft engines just for break-in, plus I'll probably burn 1-2 gallons in those this summer. 8oz of 5% fuel lasts about 6-9 minutes in my .65ci airboat whereas about 10-11 minutes on 7.5oz of 20% fuel in my LST2.