Under 72 hours? That's very generous... It's gonna be much lower...

Don't forget that players will prepare long before 2.4, and also don't forget the "20% increased XP gain up to level 20(?) item" from the recruit a friend thing.... Maybe not gonna make a huge difference, but I think it will bring down the time somewhat.

If you are 3 level 50's helping out 1 Ninja (which people will do), then you can do rush through dungeons at an extreme rate, and be 50 in no time at all...

For the more normal player, it's gonna take longer with everyone queuing as Rogue/Ninja though...

EDIT: Sorry.. I'm going with under...