I feel as though there is confusion about what we are voting on here. Release One of the Caravan Arc (i.e what Toady is working on right now, and has been working on for the last six months) is due out sometime in the near future. The entirety of the Caravan Arc is
some 9 Releases large, which is a hell of a lot of content even before you start adding in other tangents.
Let's be generous and say that each release will take half as long as the current release (ignoring for the moment that this upcoming release is technically only half of Release 1 to begin with)- that's 3 months per release and counting. 3*9 is 27 months, right there. Factor in the fact that each Release is followed by a bugfix release (for new and old bugs alike) that's supposed to be something like 2-4 weeks apiece- let's be generous and say 2. 2*9 is 18 weeks, or 4.5 months (let's round down, shall we?) 4+27 gives us a rough estimate of 31 months. If those bugfixes take closer to the 4 week side, and you've got 36 months total.
36 is the upper bound of this poll- if the bugfixes are on the long side OR if each release takes >3 months (which is only half of what Release 1 has taken, mind you) then we easily surpass that number. I feel like 3 months is reasonable (on average) for each Release's content, but that doesn't leave much room for tangents, which we know are coming because tangents are FUN! So yeah, I'd be shocked if it took less than 2 years to wrap up everything on that list, and I would not be at all surprised if it goes longer than 3. This is what Toady means when he says he expects DF to take up the next 20 years.