I'm thinking of starting a Dungeon World campaign. It seems from the experience system that levelling can be pretty quick. How many sessions would you expect to play before characters start retiring etc?
This question really heavily depends on your groups style of play, frequency of moves and duration and frequency of sessions.
Frequency of sessions: the more often you play and the shorter your sessions are, the more often you will use the End of Session move and thus earn XP at a faster rate.
Frequency of moves: the more often a move involving a roll is triggered, the more often a 6- will be rolled, thus marking XP. The frequency of moves strongly depends on how the players like to play. A more adventurous, aggressive, less cautious group will trigger moves more often than a group that is more into roleplay and making eloquent plans to deal with situations (thus avoiding lots of hack&slashing) .
Style of play: min/maxing players will level at a much slower rate than players who don't. They will optimise their stats, thus getting a +3 mod on their most used rolls quickly. They will use the fiction to their advantage and use solutions that involve moves where they have a high modifier.
That said, a campaign does not end with the PCs reaching "level 11" (there is no 11th level). In Dungeon World, you are encouraged to build your campaign around the characters, and change the world just as they do. Even if the rules state that at when they would level up from 10 to 11, you must choose one of the options presented, the character does not just vanish from the world. It migh settle down somewhere with a family. Become barkeeper in a local tavern. Two of the options even let them stay with the party.
On a closing note, my group currently has played 8 sessions at 4-6 hours each and we are around levels 5-7.
It's Your Call
The pace of levelling is a dial you can turn. From Advanced Delving: Changing the Basics p. 355:
There are some parts of the game that are exceptionally easy to change. The amount of XP to level reflects our view, but you can easily make leveling more or less rare. As well, the kinds of things players are awarded XP for can be easily changed—if your game isn’t about exploring, fighting monsters and finding treasure, change the End of Session move to reflect that difference. Make sure to share it with your players before you start the game.
You can tune it to reflect the game you want to play. Just want it to take longer in general? Boost the "base" XP requirement from 7 to whatever you like. Want the "distance" between levels to go up? Add a multiplier to the level term of the equation, as in:
level up when XP >= 7 + (next level number * 2)
If you have a preference - fast or slow - you can and should change the game to reflect that.
Take care to actually share it with your players before you start, though, to avoid violating their expectations.