I don't think there is a typical. Take the t3i. Announced in February, and 1 month later it's out.
I think the 5D3 will come quicker than later because Canon missed their usual 3 year cycle with the 1D update. Last 1D came out in Sept of 2007. That means Digic 4 processor is almost 4 years old, which is about 80 years in tech. Frankly, it's kind of embarrassing that Canon is using 4 year old tech in their top cameras.
Video in DSLRs is becoming very important. For me, it's the main thing, and all the video people want the 5D3 because it probably will have a moire free pic and more detail in the image. One of the main reasons why the 5D2 was such a huge hit was because of the video. Stills in DSLR is reaching a peak. Video in DSLR still has miles to go, and that is why I also think video will take on more importance
Canon is QUICKLY losing it's lead in the DSLR video world. The Panasonic GH2 has a much more solid video image (although I don't like the colors much and 2x crop is not ideal), and it is seriously eating in the video DSLR market that Canon created.
Canon needs to answer back to Panny soon, or else they are going lose the DSLR video market that I think is going to be very important in the future, as things converge more and more.
Remember not too long ago, people had a iPod for music, a cell phone and crappy camera for cheesy pics and maybe a cheap video camera for home movies. Well now, you can do all that with an iPhone. People are just going to want 1 camera that shoots good stills and video, and hopefully that is the 5D3.
So I think 5D3, sooner than later, especially since Canon is really late on their 3 year Digic processor update, which does not look good at all.