Here I'll get you started with a very conservative example.
"In 2006, there were about 4 million tons of conventional resources.[53] In 2011, this increased to 7 million tonnes."
If I assume 4 million MJ/kg, or 4 peta joule / ton, that's 28 peta joules.
Assuming world energy consumption of 600 eta joules, mulitply by 10 to make everyone American.
So then if we only used nuclear and didn't bother to find more uranium, we'd have a whopping 0.028 / 600 * 365 * 24 * 60 = 25 seconds worth of global reserves.
Oops. Anyway, I probably made a mistake here somewhere, but this should illustrate the methodology. Repeat for all known fuel sources, allow for some supplies that we haven't yet, adjust for whatever world population size and consumption level you think it good.
No handwaving about future magic. Always hope for the best, but count on the worst.
Fwiw what I vaguely remember is fossil plus nuclear reserves are in the order of decades to centuries, which means years or decades at American standard of living for everyone.