I will guess that the modern TT bike will have a 30-60 second advantage in a 1 hour timetrial (just a guess).

I did a 20 mile time trial in the last week on both my Cervelo R3 road bike ($4000 or so) and my Cervelo P2-SL aluminum TT bike ($1500, cheap standard wheels). I went over 3 minutes faster on the TT bike. In a side-by-race, the TT bike would have been over a mile ahead.

TT bikes are crazy fast but, obviously, the main advantage is in the aero riding position and not the bike. The interesting thing is that I ride both bikes in almost the same riding position (very similiar hips angle). I am just rotated more forward on the TT bike because of the different geometry. Both are quite comfortable.