Just to give you a kind of idea, I started dualboxing going on 3 weeks ago.

I've leveled 2 warlocks to 51 currently w/ 1 day and 20 hours, that's very casual, stopping to run instances with people and just goofing off in general. I've also level 2 rogues to 37 and 2 mages to 35 w/ about 18 hours played each, also instancing and goofing off.

So, if you sit down and just quest, you can probably hit 60 in little over 1 day played.

Also, those stickies will give you all the info you can ask for.