WotLK Arcane -> Fire question

[Selfcrusher]

I have read that arcane is the go-to spec for lower gear levels and that fire is better for higher gear levels. At roughly what point should I be looking to switch to fire? Crit at a certain %?

[Whiteman]

change to fire when you get 45-50% crit self buffed

[Audioholic]

And then go back to Arcane near BiS and top meters by several thousands.