how long will e85 last in a drum if i fill it up at my local pump before the ethanol content goes down?

 

If its sealed it will remain e85 for as long as its sealed.

 

i just dont store it at the ground.

 

Keep it sealed from moisture and it should be good. Personally, I would aim to use the entire drum within 6 months, just in case. Once in your car's fuel tank, burn it up within a month. The longer it sits, the higher the chances that it absorbs moisture.