1) How many hours did you get out of your first playthrough?
Way too many. I don't have a specific number of hours, but the length of the game was almost exhausting. The amount of actual content is comparable to Baldur's Gate 2, but the slower pace of combat and large number of side distractions made it take about twice as long as it usually takes me to get through BG2.
2) What percentage of the game content do you think you experienced?
Almost all of it. At a certain point toward the end, the game refuses to let you continue until you've found X amount of Macguffin stones, which forces you to either go around finishing sidequests in pursuit of more or spend two hours alternating between whacking a door and repairing your weapon until it finally goes down.
3) How replayable would you say this game is?
Not very. Enemy placement is predetermined and there don't seem to be any "evil" options that would unlock different story outcomes (that I'm aware of, at least), so while you could probably play around with different approaches to combat, I wouldn't say it's anywhere near as replayable as many other RPGs. Especially since many are likely to explore the magic/marksman/man-at-arms trees fairly heavily in their first playthrough.
That said, Larian
might add some more companions at some point. That would be worth playing through the game again for, because the two NPCs that are in the game now are both interwoven pretty heavily into parts of the story. Otherwise, the game is a lot of fun, but it's not really one of those "start a new playthrough immediately after finishing" kind of games. Definitely worth re-playing a year or so later when you've forgotten enough of it for it to be like a new experience, though.