Shelf life of a substance?

"Shelf life" is based on the rate at which certain reactions occur in the oil - mostly oxidation. Old oils go rancid because the oil molecules get oxidized, forming compounds with an unpleasant odor and taste.

Different people will notice the rancidity at different levels. Some people might be extremely sensitive to a slight rancid taste in the olive oil, but have a harder time noticing it in hemp oil (especially if they're not familiar with hemp oil).

Mixing two kinds of oil probably doesn't slow the reactions affecting either one. It might delay the onset of noticeable rancidity, because the concentration of the "rancid molecules" from the hemp oil will be lower in the mixture than they would in the pure hemp oil -- even if they're being produced at the same rate.

I'd guess that mixing olive oil and hemp oil will probably give a shelf life somewhere in between (longer than hemp oil, shorter than olive oil). The length probably depends somewhat on how much hemp oil is in the mix. 1:1 hemp:olive would probably have a shelf life a little less than 1.5 years, because at that point the hemp oil would be "very" rancid.

Unfortunately that really is just a guess...

Final note: Vitamin E is a fat soluble anti-oxidant. Adding a small amount of vitamin E to the mixture might extend the shelf life substantially. (There's a number of other ones that get used also, notably BHT; "all natural" is important to some people.)