Here's one interesting tidbit on how long it took him to complete:
Read the full interview at GamesRadar.
In terms of the time it takes to finish, how does it compare with others in the series?
That's a hard question to answer really. I finished the story mode in 24 hours, without skipping cut scenes, but I literally blitzed through and hardly took on any side missions. If I was to play it again (which I will be, right on launch day!) I'd say to wrap up the story will take at least 40 hours at a normal pace, plus another – say, 25? – to wrap up the remainder of the missions. In truth though, you could be paving the streets of Liberty City in a year's time and still not have discovered everything. Plus, if you're a 360 owner, there's the promise of downloadable content to come...My advice is: just kick back, take your time and breathe every fresh sight and sound in, don't simply rush through just to finish it before your mates. I wish I'd had that option!