I'll occasionally pace a group of roadies to see how long I can keep up with a road bike on a mid range mtb with off road tyres. Its not usually all that long as the effort to keep an MTB rolling at 40km/h or faster on the flat soon wears me out. I'd rather cruise along at 27-30km/h and enjoy the ride. At that pace I keep going for about 4 hours without stopping, or until I start to run out of fuel...