So, I took few numbers out of Google Video search.
After adding up all those minutes, it became obvious that I may not live long enough to watch all of them :P
In fact, it will take much-much longer...
So here are calculations:
113000000 * 3.59 minutes = 771.310869 years
75300000 * 4 minutes = 572.679355 years
9990000 * 21 minutes = 398.879156 years
771.310869 years + 572.679355 years + 398.879156 years = 1742.86938 years
So the final answer is that there are currently at least 1742 years of video available to watch.
And that's always counting.
BTW, if we're talking about increasing numbers, Gmail is 7074 MB now.