It’s not linear. Levels 55-60 took me a week in Vanilla. 45-55 was almost 2 weeks, too; since the quests and dungeons at that level range are sparse. 20-40 was about a week. 1-20 was about a week and a half.

Classic took much less time than Vanilla.

While four hours would be fast for a new player, 1-10 is very doable in one sitting for those familiar with the game. Early levels are fast; the bulk of those 30 days are later levels. The 50’s in particular are a slog.