Or at least, that's how long it took one game-guide writer.
The original 'Borderlands' was a game built on replayability. It didn't have the longest campaign, but thanks to DLC packs and the option to replay and fight harder baddies with better loot, players got a lot out of it.
'Borderlands 2' is a bit different in that the campaign is going to take enough time to satisfy even the most demanding of players. According to Gearbox, a recent playthrough by a game guide writer took an impressive 58 hours.
Your mileage may vary of course, but nearly 60 hours of gameplay on one playthrough with one character is JRPG like in length. As long as those 60 hours of play are good, we're in for one potentially great game.