Years and years and years as long as the bottom of the drum hasn't become rusty and some how contaminated.
But having said that I would prefer to keep any container you use full that is several 5 litre containers instead of 2 litres of oil and 18 litres of air.
That might not make any diferance but i'm just guessing it would.
But I would shake the drum around a bit before pouring any out! surely there has to be some kind of settling of the oil after years of just sitting still.