Well from what I remember of previous Rockstar games (since GTA IV) hands on walkthroughs were available to press before release, although I'm not sure exactly how early, probably a couple months or so. Also I've gone to the last 3 PAX East events in Boston and Rockstar had a big showing at all 3 of them. First year was RDR which had a hands on playable demo for everyone to play (behind closed doors). Next was LA Noire which we were able to see a walkthough (not hands on) of a case while a Rockstar rep explained the game mechanics to us. Last year was Max Payne 3 and we got a hands on playthrough of a level.
PAX East is March 22-24 this year. I fully expect Rockstar to be showing off GTA V at that time. I think it's safe to assume press will get to see a walkthrough of it before then. I also expect their PAX East showing to be a hands on playthrough like RDR and MP3.