In an interview with VideoGamer.com, a Rockstar employee speculated that the latest installment of Grand Theft Auto will take about 100 hours to complete, give or take. That estimate is based on whether or not you answer the phone. No, not your phone, silly! Niko's cell phone! Says the mysterious, unnamed Rockstar employee:
"It's harder to distinguish between main and side missions - there are loads of both. But there are more main missions than side missions. It's not as clear cut as it used to be because of the phone."
Looks like if you're the type to avoid side quests and gun for the ending, it'll take a lot less time, but half the fun of the GTA series is all the little stuff, so it looks like most of us are playing it for the long haul.