Approximately 60 hours, to be exact. Brady games, publishers of the Borderlands 2 official strategy guide, recently played though the entire campaign, and all side missions. It took them 58 hours to do so, and this fact was recently announced during a Destructoid.com live show.
A post was made about the matter on the official Gearbox forums, and a Gearbox employee confirmed the number.
Playtime will always vary depending on play style. Some may finish the game faster some may not.
The time that was quoted was how long it took Brady Games to get through all the main story and side missions on the first play through.
I guess that is a good indication that the guide will be nice and beefy
To add to that already crazy sounding length, keep in mind that this was the time for the FIRST playthough. Borderlands 2 will have at least a 2nd playthough option as a new game + feature, and developers have unofficially announced a possible 3rd playthough after that.
And that doesn't even cover all the fantastic DLC that will no doubt be released.
For reference, it took me over 30 hours to complete the original Borderlands, which included some co-op replaying of missions I'd already completed, but did not include a large number of side missions. By the time I put over 50 hours into the game (across two characters), I was knee-deep into my second playthough, and I was still only a level 38 out of 50. Borderlands 2 has been said to feature 60 experience levels, adding even more replay value.
This--along with the recently announced infinite upgrade system--is a good indication that once again, Borderlands will be one of the best values in gaming.