The 3E calculus for leveling speed, IIRC, was based on some market research done by WotC. It was approximately this: given a campaign constituted of single, weekly four-hour games, the average group could expect to advance from level 1 to 20 in approximately one year of play time.
So, what FranktheDM said for 4th edition, except it's now approximately 1 to 30 in one year of playtime. It remains to be seen how many encounters/session that will break down to, etc.