I finished the main missions yesterday and I'm only 65% in Absolute quality game. I very rarely play anything other than Fifa so for me to say this is good...
Same Spoiler alert: [NSFW]I couldn't bring myself to do anything but the other two options. Also, surely killing one of them off would mean some of the remaining tasks wouldn't be available?[/NSFW]
Depends how good you are, how much pissing about you do and how many side missions you carry out. I'd guess someone that's played the other GTAs and just does the main missions would do it in about twelve hours.
20 hours I reckon for the main missions. I'd be surprised if that's all you managed though as it's so easy to get distracted. When I finished the campaign I was at 79% game completion but haven't gone back on since. I will though. Just need a break.