First, you state a few things that aren't quite right in your question. While the view that's generally talked about is that Phobos and Deimos are likely captured asteroids, dynamically it's a pretty difficult problem (you generally need a third (in this case fourth?) body to take away the extra energy, and it's hard to get a circular orbit around the equator). See for a bit more on that.
In terms of Phobos' demise, there are two things that make this problem very difficult to estimate. First, Phobos' orbit evolves as it orbits around Mars, so you can't just take a linear approach and say, "It's moving towards Mars at 18.3 cm/year so it's going to hit in about 50 million years." It's more complicated and non-linear.
But besides that, there's the Roche Limit to consider, whereby the moon will break up due to tidal forces before it would actually hit. The problem there is that Phobos is already within the Roche Limit, meaning that it's only being held together now by the physical strength of the rock it's made of. And since we don't really know what it's made of inside (though we can make educated guesses and I'm sure there are models out there for its strength), these unknowns make it somewhat difficult to estimate.