This will depend on how long you let the bot run each day. I normally quest until 60 and then Arch from 60 until 85 and it normally takes me about a week to ten days (depends on the squishiness of the toon while questing) at 10 to 12 hours a day to get to 85. I arch instead of quest at the higher levels because I find questing glitches more in the higher level...perhaps because of all the little click this and pick up that quest lines. With Arch I level to max arch skill in Eastern Kingdom, move my toon to Kalimdor until 80, then hit Outlands and Northrend until 83 (to avoid Cata zones) then back to Kalimdor until 85. I like Kal as there have been no sites that I have had to blacklist at all whereas there are several in EK that I have to watch for due to trees. In Kal I can afk overnight without issue, EK I have to check on every 30 min or so.
When you hit 80, consider using GB and gathering herbs/ores to level. It is slow going but you can make alot of gold while leveling to 85 that way.