A game is going to have to be SPECTACULARLY good for me to want to do a 150 hour playthrough. As an example, I've recently been playing Pirate Creed, and I was enjoying it a lot, I think it's a great game and it has a good variety of different things to do, but after a little over 40 hours, I got very worn out and had to stop. Now, I'll eventually go back to it, but if it takes me more than another 30, maybe 35 hours, I'll likely stop again (knowing myself) and every time I stop again and the game is not yet finished, the likelihood of me coming back to finish it decreases considerably. Crazy as it may sound, there is such a thing as too much content.
Of course, a different way to play is to only do some quests on a playthrough, with the intention of doing different quests on a different playthrough. There are a couple caveats with that, though. First, the game has to be good and interesting enough, and allow for different enough builds to warrant multiple playthroughs. Second, RPGs, in particular, tend to lend themselves to players doing every single possible quest in a playthrough, because of the desire to amass maximum exp and make the character as powerful as possible.