a) I think decline will start at latest by 2020, probably a year or two earlier. I came to this conclusion by looking at past declines and current regional production data. Exports decline has already started in about 2005 - oil producing countries consume a larger part of production by themselves. Major oil price fluctuations can be damaging, because too high prices cause demand destruction and too low will bankrupt marginal producers.
b) We don't know what type of decline to expect, but it might be steeper due to EOR (http://en.wikipedia.org/wiki/Enhanced_oil_recovery) techniques depleting the fields faster.
c) We don't really know how large part of the global recoverable supply we've consumed, because it is likely several OPEC-countries have reported exaggerating reserve data in past in order to increase their production share.