How long to be good at guitar?
If you're a regular reader of this blog, then you'll know by now that I'm not a fan of the word "good." Why? It's a judgement. Judgements require an immense amount of energy to maintain. I personally feel that judgements get in the way of learning guitar. To me, the process of becoming familiar with how to play guitar is far more important than becoming good.
That said, I think it's fair to address your question of how long it will take to be good at guitar. There's no point in you doing this unless you have an end point to shoot for. If your end goal isn't defined, if you can't define what "good" is, how will you know when you're there?* Let's throw some ideas around.
What is your definition of "good?"
First off, how unemotional can you be about being good? Achievement-based goals are reasonable goals that can help you define what good is to you... Knowing chord progressions, strumming, and maybe losing count of the amount of songs that you know. Start by getting a general idea of what skills that you think a good guitarist ought to have.
There are some things, some goals, you may want to avoid when you are defining what good is. My first suggestion is not to define what good is by comparing yourself to anyone else, like a comparison-based goal. My second suggestion is not to allow a sense of urgency to creep into your definition. Guitar takes time to learn. My third suggestion is to resist the urge to put a deadline on getting there. While we're on the topic of time...
Okay, what about how long?
How long do you expect it will take you to learn the skills you've come up with? If you 're a complete beginner and you want to learn atonal classical guitar music, than you're going to be at it for years. If you are a beginner and you defined that a good guitarist can strum, sing, and enjoy music, then it will take you far less time. What, in your opinion, can a good guitarist do?
Definitions are necessary, but I'll give you a secret: If you enjoy the adventure of learning guitar more than just becoming good at it, you won't care about getting good. My big suggestion for you is to throw out the words "good" and "bad" altogether, and instead focus on the process of learning guitar.
Also, if you focus on urgently becoming good on guitar, you'll overlook the simpler and more delightful moments of learning. Learn to enjoy the process of getting stronger and more skillful at guitar, and you're skills will grow on a daily basis without very much effort. Set unemotional achievement-based goals, and use them to direct your daily attention to what's important. When your skills catch up to your desire to play music, you're life will change. On that day, you won't care about being good.
You'll care about having fun.
*Professional musicians who actively practice each day don't usually have a clear definition of what "good" is. They are willing to suspend their belief that they can get to what they've defined as good, and get more into the process of learning and practicing. There's nothing wrong with just enjoying the process of practicing and performing just for the sake of practicing and performing.