Not long but it depends on how many polys you want and the method you used. If you're using the new beta it would be less than 45 minutes...probably a half hour since it's primarily tube based. You could even get away with using symmetry in the new beta since it doesn't require 100% symmetry on the reference to work, which would cut the amount of time down. If you're using version 1, alot less is automated so you're probably looking at a full hour to maybe an hour and a half if you want to capture every wrinkle.

 

The beta of Topogun 2 has these nifty tube thingies...they remind me of loft curves in Rhino3D. They will conform to the mesh and will generate any number of polys you set to them. Once done, you can then build off them for the rest of the shirt. Alternatively, you could just paint the topology on top of the reference mesh and then bridge or manually combined polys to match up the various surfaces generated. All of this mentioned could be done in a few minutes. Now if you want to get more detailed in the topology generated, things will obviously slow down a tad but the surface itself is simplistic so there's little need. The only areas you'll probably want to do manually are the edges of the shirt and the collar. I really don't see it taking longer than 45 minutes and even that is longer than it probably would take.