Since I've done it four times in recent years, I shall pipe up.
I'd figure at least two days, or more to ride all the really good roads in the area.
Spearfish Canyon - the Moonshine Gulch Saloon in Rochford - Newcastle to Custer to Lead to Deadwood.
Spearfish to Bell Fourche to Devil's Tower and Hulett, WY. (This SHOULD be done VERY early on Wednesday).
44 from Rapid City, 385 & 16 to Hill City. 244 to Keystone and then all the 16, 6A, 87, 244, 753 and 359.
Note the 16 section that runs south from 244 to 359 SHOULD be ridden from the south to north!