The factor you're looking for isn't time. That sounds funny, doesn't it? But it's true. You're looking for a divergence in the human gene pool to the point where it would be incapable of reproducing with the previous point. And to that end, time is one factor among many, and not even the most important one. There are species on this planet which have basically remained stagnant and unchanged for ten of thousands of years, and species that have radically altered between the same set of ten of thousands to produce a dozen species that can't interbreed.
What you're looking for is environmental pressures. Evolution is always in response to a stimulus, and time isn't that stimulus. Predators growing better, new and different ecology, a random mutation which makes the animal far more suited to it's current predicament and thus possesses a massive advantage towards reproduction - those are all things which affect evolution, and time only affects the last one. True, the current model suggests that we need the last step (random mutation) for evolution to occur, but the time it takes to mutate seems like a limiting factor, i.e. will not happen in less that this level of time if all the other criteria are met.
In other words, take your example. The humans gene pool will alter based on whatever climate that the planet contains - if it's a warm planet, it would develop to be similar to humans in warm places on earth, and so on and so forth with things like cold climates and damp climates. To truly have a radical shift in the human genome, you need them to live unaided in an environment that human aren't capable of living in now unaided, and them having adapted to it, presumably having sped up the process with genetic engineering of some form.
It's possible that, regardless, there will be a buildup of thousand upon thousands of small random mutations within a gene pool that will eventually lock everyone else out, but that's a time scale of hundred of thousands of years, if that's even possible to occur without some form of outside stimulus. (This is based on the fact that there doesn't seem to be a problem with interacting with uncontacted tribes, and some of these have possibly been in isolation since the dawn of man.)