In game time, it takes at least as long as all the story missions and side missions last, plus up to another 25-75% to allow for procrastination and exploration.
In real time, it probably takes weeks to months, as I don't like to rush my games and I play them at a leisurely pace.
Progressing the story is always the last thing I do and I love spending a lot of time just experiencing the game world without necessarily achieving anything significant.
As well as having a huge backlog of games between my PC and 360, I don't always spend all my free time playing games, let alone playing one game to completion, so it's rare that I'll complete any one game within a few weeks of it's release.
Game completion is very low on my list of priorities and is not a significant factor in my enjoyment of games, which is probably why I spend a majority of my game time playing "endless" games like Mount and Blade: Warband (I have 2 games in 2 different Mods running at the moment), Forza 3 (a game with no real end, that takes a long time to complete every single event, and a game that I can get distracted from progressing in because I love tuning and designing new cars) and Red Dead Redemption (a game that I've completed once, but now I spend most of my time in it at a point before the major change, just hunting, bounty hunting and exploring).
Needless to say I disagree with the attitude of the poll options, none of the games I own are difficult or challenging enough to prevent me from "beating" them in the time it takes for the missions to end.
There's nothing particularly good or noteworthy about completing a game quickly (apart from dubious Kudos), nor is there anything wrong with taking time to enjoy and experience a game.
I think because I don't have a "rush it, beat it, trade it" attitude towards games and I don't care about achievements or keeping up with the Joneses, there's nothing of monetary or prestigious value to be gained by completing a game quickly.