The entire process is going to vary greatly based on a lot of different factors. Someone who's "played through" dozens of times is going to have a far faster time than someone who hasn't. Already being familiar with the classes, software, macros and multiboxing in general will decrease the time to level. Having RAF/Heirlooms/Max level guild/rest bonuses or not will impact it. Knowing various leveling tips like when to do what dungeon and when to go out and quest/grind/etc will shave time off.

I can't speak to my team since I've been playing the same team since Wrath {some of the toons being rolled back in the later half of BC even} so their actual time /played wouldn't remotely reflect time to level, but I have gone back and continued old teams to waste time. Playing the slowest, most time wasting fashion I can {generally doing every quest in a zone until they go grey, even if it's a gathering quest, doing dungeons only once if a quest sends me there, etc} I still make it through an expansion a weekend as a general estimate. Then again, I can probably boost a new toon or team in a single weekend.