Just what I hear from academics (correction: I just spoke to someone who said I was thinking 40 years, not 20 years), and I admit that is probably a conservative estimate. Of course, its difficult to estimate - most middle eastern countries have about 80 years of proven reserves at current production, but energy requirements are likely to expand significantly - more than double over the next 10 years, I heard recently on tv.
On the other hand, there are supposedly a ton of untapped oil sources in other parts of the world (but perhaps not in the UAE), so it all depends. I do know that the UAE is trying to diversify its economic portfolio, getting into financial and services industries. I wasn't trying to say that oil would be gone from the world in 20 years.
And of course, we're not sure when the next energy breakthrough will be. In 20 years, we might have tons of oil remaining, but due to alternative energy sources, that oil may not be worth anything. Or it may be hoarded and worth more than diamonds. Who knows.
Although, if the UAE sold oil at the same rate that Saudi Arabia does now, its provable reserves would expire in 20 years, according to wikipedia.
Sorry for bringing this convo way off point. After her finals I can talk to my gf (she's in grad school for energy) and get this sorted out. It's really not my area of expertise.