BoardGameGeek says Le Havre takes about 150 minutes, but I'm curious if the time varies with the number of players as it does in Puerto Rico and Agricola. Approximately how long does it take to play Le Havre with 2, 3, 4, and 5 people?
It's 30-60 minutes for player, very much like Agricola. This is in the Wikipedia article, but I think it says it on the box too. The rules (zipped pdf from the publisher) have a time/rounds per player table with the conservative estimate (60 minutes per player) for the full game, along with a ~30 minutes per player estimate for the shortened version (20, 45, 120, 130, 150 minutes).
The rules say this (for the long version. I don't play the short version and don't know anyone who does).
I've played about 20 games with at least a sampling of all player sizes. I don't think Solo is worth considering time-wise. You'll spend however much time you want, nobody will be uncomfortably shifting in their chairs or humming the Jeopardy theme song to hurry you up.
From experience I can tell you that a 2 person game runs 90 minutes if you are both fairly familiar with the game and have a good setup procedure. If, when you last played you dumped all the components in the box without organizing them, add an extra 30 minutes...
Each player after that is going to add a fairly static amount. Perhaps 45 minutes.
This assume mythical standard players of course! :) If anything Le Havre is going to bring out the worst in the players that like to know they are making the right move by calculating out as many possibilities as they can.
Each additional player is going to add a good amount of time to the game. Le Havre is also difficult to plan ahead. Your options can really shift depending on what the other players do. Unlike say, Agricola, where you may know on certain turns what you have to do (like acquire food) or Puerto Rico (time to SHIP!), Le Havre doesn't really have forcing actions. There are always options.
The player right in front of you can easily open up options by buying a building unexpectedly, or moving off of a crucial building. They can also screw you up by sitting on a needed building for a third dang turn! I find that these sorts of things happen quite often which doesn't reward folks for thinking ahead (at least in terms of reducing the time it takes to play the game). It's likely that folks with more experience than me will see more gains here.