It's going to depend on how fit (and stubborn) you and your mates are. Remember when you're touring that you're carrying plenty more gear and by the road repairs are more likely to be needed (three broken spokes in one day on separate occasions set us back about two and a half hours once). But on the other hand time isn't too much of an issue unless you want to do lots else during the day. Don't forget that you'll be doing it day-in, day-out as well which will take it's toll.

My advice would be to give yourself plenty of time to enjoy the trip rather than having to watch the odometer the whole journey.