Untill the next oil technology breakthrough. Oil also has major economic limitations on how PROFITABLE it would be to drill it all out that fast, which is why it's never happened and is why we're not likely to "run out" of oil for a good century or more (though we might see more short-term shortfalls as we have done in the past).

 There is some similarity of oil and bitcoin though, in that they are both limited in available product TO "mine" - the biggest difference is the artificial limits on how fast Bitcoin CAN be mined, but it still makes sense for a big miner (be that a manufacturer or not) to push to get as many as it can now BEFORE OTHERS BEAT IT TO THEM, and makes ZERO sense for a miner to try to "hold back the hashrate" - there are too many folks that WON'T collude to make that a viable strategy, unlike oil production where the number of significant producers is strictly limted by the HUGE PRICE OF ENTRY and the limitation of most major oil producing nations having nationalised their oil and access to it (The US and UK are the only significant exceptions to that last, and even the UK has been semi-nationalised for a lot of the time it's oil industry has existed).

 P.S. - who do you consider the "big three" - there's more than 3 makers still around, though a few don't sell to the public any more.

 D'Beers (sp?) is a very different situation from oil. One company in one country controlling the majority of the "reserves" of Diamonds is in no way comparable to the number of producers and countries in the Oil industry - even the Saudis and their "largest proven reserves of any country in the world" are nowhere near having the kind of control D'Beers has over diamonds (witness the current "oil price war", which certainly is NOT maximising profits for anyone).