Just long enough to have a really good time playing The Division and maybe be left wanting more. Not enough time to realize that there's absolutely nothing worthwhile endgame-wise. If you haven't played it yet, I encourage you to take advantage of this opportunity because it's a beautiful game with fun mechanics and some challenging levels. I wouldn't recommend buying it, but if you find that you love it, it goes on sale for like $15 pretty often but I'm just warning ya, it will feel old after a short while. It's a really fun couple of weeks at first, though. That's really the most disappointing part: it has a ton of potential but the guys running it either don't have the resources or the expertise to make it a replayable game.
AAA games should provide free trial like this one, since there is no demo or trial version of these 60 dollars games nowadays, hopefully this will some give insight and live up expectancy of the promised gameplay and reduce piracy rate and encourage people to buy legit version