This is very hard to estimate -- 1) the oil companies may find new sources 2) they may find that existing beds aren't as productive as projected 3) extraction m

This is very hard to estimate -- 1) the oil companies may find new sources 2) they may find that existing beds aren't as productive as projected 3) extraction methods could improve 4) the rate at which we are using it could increase or decrease. And of course the question at any given time is not how much oil exists but how much is economically worthwhile for someone to extract. But yes, the supply is certainly finite and between a few decades and a few centuries -- and while this seems like a small amount of time, it's not in comparison to how long it took for these deposits to form (tens to hundreds of millions of years).