Pure solo on my enhance shammy was about 12 days /played (first char in the game). (300 miner/engineer)
Pure solo to 60 on my mage was about 8 days /played. (no professions)
Pure solo to 60 on my hunter was about 7 days /played. (skinning only)
My 4-hunter group hit 60 in about 10 days /played. (no professions)
My 4-priest group hit 60 in right about 12 days /played and 70 in 18 days /played (tons of professions on these).
My 4-mage group hit 60 in about 10 days /played and 70 in just under 16 days /played. (herb/mining/skinning only)
My priest+locks group is sitting at about 9 days /played at 51, so I imagine it will end up around 12(60)/18(70) or so for these guys, though I could be off as I'm doing professions on all 5.

Short version is that you'll save time leveling if you do it all at once versus trying to break things up. And having a 70 "powerlevel" you is not as uber as you might think.