I know you posted this a long time ago but I wanted to reply so others might have the answer.
A colleague just ran a test of about 7 sq miles and it took 5 days, only 155 days were analyzed.
In the same spirit:
I believe it generally depends on the complexity of the terrain and the resolution of the original elevation file.
Ones i recently ran were taking around two hours for an area about 2/3 the size of Wales (186.2 km x 134.45km) at a DEM resolution of 50m (38.20 MB file).
If you have the possibility, use GRASS. Faster and better results ...
Retrieving data ...