I feel it was a bit short, I don't know if it just feels that way because the time we spent with one character in previous games has been spent with 3 in this one, which makes you feel a bit less invested in the characters or if it actually is short, I've got around 80% so there are plenty missions I could reply to score more highly on, but also I didn't do all the side missions fully. I think it's brilliantly written, I just think a few more missions with each character, in paricular Franklin and Lamar for instance would have been worth while.
Either way when we look back at things like BOGT and TLAD , I'd imagine there will be a lot of expansion headed our way if online isn't your thing. I'm just starting out online doing races etc but I can see it getting a bit boring/repetitive as I haven't really enjoyed any of the missions I've tried. Maybe with the introduction of heists etc that may change.
I'd say it took a lot less time to complete this than it did gta IV though.