I pretty much was in the same situation as you, having bought the Orange Box and, having nothing else to play, wondered about playing Half-Life 2. I ended up trying it out less because of what people said about it and more because I owned it and I at least want to play all the games I own.
Here's what you'll probably find out if you play it:
It's overhyped. It isn't the perfect game everyone makes it out to be, and if you don't go into it with the right mindset about silent protagonist stories (ie, that you have to develop the personality yourself, which is a bit difficult because the characters from the start all seem to act like you've got a personality already set), you're probably going to be underwhelmed with the story.
That being said, the voice work is solid and while the character models are on their last legs in terms of graphics due to the decision to go realistic rather than stylized they do get their points across and you'll soon be able to see past them.
As for gameplay, it's solid, but nothing extraordinary. You're given a large variety of weapons, which have enough of a kick to feel right, and most enemies react properly to "bullet to the face," though there are certain ones (especially in Ep2) that don't seem to feel anything when you shoot them.
Now, a word of warning, most of the first game will feel oddly similar due to how much of it has appeared in countless other games (physics puzzles, the desert level, etc.). Like with a lot of "legendary" games, you won't really experience the same thing as the original players where this sort of thing was more impressive.
All this being said though, there's a lot of fun to be had (especially on the final level), and it still manages to be a good game despite its age.
It's not something I'd drop a game I'm currently interested in to play, but it's definitely one that I'd play if I had the time.
As for the episodes, the first is really fun, though the scenery won't be that impressive. The second sacrifices a bit of the fun for the sake of better set pieces and better environments (when you get out of the caves). Both are a bit longer than most DLC/episodic games you'll see, but they still seem to end a bit too early (part of this is the massive love of cliffhangers the games have that even rivals that of Assassin's Creed).
In the end, Half-Life 2 and it's episodes are better than most FPS games, though they're nowhere near the games fans make them out to be. They're effectively the Kingdom Hearts of FPSs (amazingly impressive at the time that spawned a fanbase that hyped them up to a godly level which spawned an equally vocal hatedom).
As for time, Half-Life 2 I think clocks in at around somewhere between 6-12 hours (depending on how you play and how good you are) while the episodes go from 2-6 hours in length. So that means you'll probably see 10-24 hours or so.
