I have a 19' with a 20hp on it and it takes about an hour from Linesville marina to the causeway (we only move about 8mph though according to the gps), so I would guess at least 1 1/2 hrs from Jamestown to Linesville if your doing the 10-12mph on your boat...I can't remember what it took the last time I actually made the run down the whole lake, its been a few years (I just haul the boat via trailer if I want to fish the other end)...
*ADDED- I used the ruler on Google Earth and went from the **** to the spillway and its 14 miles, so it would be just under 1 1/2 hours at 10-12mph...