Today I found my dad's old guitar, which he hasn't played in years, opened the case, and instantly fell in love with it. It's in pretty good condition, as far as I can tell (not that my opinion is a very good one, as I know almost nothing about guitars, or any string instrument for that matter.). I've been wanting to learn another instrument for a while (I'm a flute player), and I was wondering how long it takes to learn to play a guitar, and how much I can learn from reading and videos, and what I need to learn from a live person.

You can learn from books and videos, it just takes a bit longer. I tried that for a couple months, I progressed a bit, I was only like 8 or 9 though, you may progress further or less depending on your learning rate. You could try it like that since you are a flute player, already knowing a bit of music theory helps a ton. But if you get a guitar teacher you learn a lot more, you can ask questions to your teacher if you're confused (as you can't with books or vids) and you tend to go right along learning stuff every time you take a lesson... I recommend a teacher, but if you don't have the time or money for one, you can stick with books and vids.

Unfortunately, it depends on what your definition of learn is. If you mean to play 3 blind mice then 3 weeks should suffice, if you mean the Rodrigo then several years would be more realistic.