Depending on how you're counting, and how many of the game's locations you end up exploring in total, you're somewhere between 1/3 and 1/2 of the game complete.
Like folks have said, the Great Souls constitute what is essentially the front half of the game. In terms of hourage, though, it might vary. I felt like the game post-Great Souls flew by, and spent a majority of my playthrough just working through the early areas.
Oh one other thing: Armor is not the be all end all. You can upgrade your armor as high as you want, wear the heaviest nastiest stuff around, and you will still feel it when you get hit. So don't kill your equip weight thinking running around in the heaviest crap around will make you take like half damage.... it won't.
By the middle of the game I'd totally shifted my playstyle away from a tank-build to an agile fast-rolling build. Dodging trumps tanking.
Putting points into any stat has diminishing returns after 20 points.
That's definitely the soft cap for stats, but it's not the whole story.. From 20 points to +/-50 points invested in any stat, you're going to get diminished returns, but not diminishing. Truly continual diminishing returns begin after the 50-point mark. Which is just an insane point investment into any single stat anyway.
Depending on your style of play, @nocall, you're going to want at least one of your stats to be invested in beyond 20 points. If you're melee-based, that probably means STR or DEX (or both?) and VIG.
@lackingsaint: The Drangleic armor is more of a stamina nerf trap than anything else; it's not very good in general.
How do you mean? Like in terms of making the player Fat-Roll? I've always made sure to put stats in Equip Load so at the very least I can Normal-Roll, and honestly half the armour i've found either weighs around the same or more than the Drangleic set (with worse stats, mind you).
Another little question; is the Lost Sinner supposed to be this difficult? I beat Forest of Fallen Giants, Heide's Tower, No Man's Wharf and most of The Bastille when I found her zone, so it was fairly early-game, but since then i've beaten Grave of Saints, Shaded Woods, Belfry Luna, Huntsman's Copse, Harvest Valley, Earthen Peak, Doors of Pharros and Brightstone Cove Tseldora, and I STILL get my ass handed to me whenever I try to fight her. I'm using a sword-and-shield build with no magic and a DEX focus, with medium-roll, some heavy armour, and a 100% Phys-Block Shield. At this point, this lady feels like my Everest, the boss for me to strive for while I complete the rest of the game.
Lot of folks are saying Lost Sinner is one of the hardest bosses in the game. You're not the only one.