At least lvl 58-60 , more like 70, so it make it easy to hang around picking herbs without fighting mobs and just lvling it up, and of course u have to be 58 to go Outlands.
Basically, its very easy to switch to another prof at 70 if u dont mind loosing previous one and definitely know what u want because u probably have enough money to buy low lvl stuff at AH or, as i said, to go to low lvl areas just for grinding mats without loosing time for fighting. My friend had herb/mining till lvl 70 just for making money - selling everything through AH, and once he bought epic flying he switched to tailoring and lvled it up to max in just 1 week  - so long only because he was grinding mats by himself, just fair play as he said ))