This answer to this question depends on quite a few things. When you learn something new, your brain produces something called mylen, which insulates the neurons in the brain so you can perform a task, like playing guitar quickly.
The good news with mylen is that you're never too old or young to produce it. Your brain produces it constantly as you perform tasks. Now that you have a little encouragement, it's time to look at the 2 biggest factors for you as you learn.
1. your passion
2. the information you use to fuel it.
They both need to work together.
First of all, notice I didn't mention natural talent. Sure, some people are naturally gifted in hearing and understanding music a certain way; however this factor is NOT the greatest indicator of how long it will take you to learn. This factor plays directly into your passion of playing guitar. There are many motivators in playing guitar, and this is only one of them. If you don't care about guitar, then you're not going to want to stick with it. Find what inspires you and plug into it! Feed your passion! Want to learn, and you will learn quicker.
Secondly, if you spend your time trying to read music notes, or trying to conquer bar chords too early in the game, you're going to waste so much time (even if you are practicing for 8 hours a day). 1 hour of practice of solid information is worth much more than 8 hours with bad information. Find a good source of information as well as inspiration. Guitarmann provides a structured approach to help you accomplish your goals of playing guitar by creating a step by step approach.
With the right passion and information, you will be playing though your first few songs in a few weeks. If you stick with it, you will have a solid understanding of keys, chord families, how to play most every chord you need, and ultimately how to play rhythm guitar to 90% of songs in ANY key in about 4 months time. Tack on a few months to have a solid understanding of how to use electric guitar to solo all over the neck and play complement chords.
If you stick with it, you will learn. No question! As always, guitarmann is here to help. If you need help along the way, just ask in the forums!