Leveling from 80-85 will really depend on you the player. I say it that way because several factors will play into it.
1 - Do you "read" the quests? If you do, if you're interested in the lore behind it -- which some quests do get into -- then you're going to take a bit longer leveling.
2 - Do you know someone who played the beta version? If you do, and can tap that knowledge base once Cata is live, it may take you less time because you're going to have a guide to help you along. I know my guild is excited that I'm playing the Beta and have already "deemed me" the Cata guide once it's live.
3 - Do you use a lot of addons? IE: Quest Helper, Carbonite and etc. If you do, it may take you longer to level or less time depending on how quickly those particualar addons are updated. On the beta, we have no addons, (and I don't use them live anyway), so for me, I running through levels pretty quickly. Yesterday in about seven hours of play on the beta, I went from 20-27 on my Goblin. Now, bear in mind that the xp needed per level on the beta and what it will be ultimately on the live version will vary, so...
4 - Do you level primarily through Quests or Dungeons? Now, I've known players that go both ways -- each has it's own pro's and cons mind you -- so that will make a difference as well. If you level primarily through Quests, I've found that in Beta there are plenty to get you from 80-82 and ultimately 85 once the rest of the content is available in the Beta. If you level primarily through Dungeons, you might not get there as quickly due to the fact that at the moment there's only about 5 to work with vs. about 10 that came out with Lich King, so keep that in mind. You may need to do a combination of both in order to level relatively quickly once Cata is live. Up to you.
Hope that helps.