This is an extremely common question I get from users, and so here is a bit more than you wanted to know about the answer:
Short answer: there is no reasonable answer I or anyone can give you.
Long answer:
What is fluent?
This is actually the hardest thing: being fluent means different things to different people! Full fluency (as in, being able to produce/consume *anything*) is more or less impossible, even for native speakers. There are many topics that each of us would not be able to discuss/comprehend in English (or your native language).
Putting that aside, some people do want to have a "well-rounded" level of comprehension that lets them exist within Japanese without having any issues, but even this is hard to pin down. For example, what would be required of you (in terms of knowledge) is radically different if you are a student in a high school/university, working for an international firm in Japan, living out in the country, or doing part-time jobs in the city.
I, for example, feel comfortable in my day-to-day life in Japan, and I feel pretty comfortable with business Japanese, but if you put a science textbook in front of me from a typical high school, it may as well be a different language. But, I could talk with someone for hours and hours without needing to look up anything.
So I might be considered fluent for people aspiring to a certain type of interaction with Japan/Japanese, but far-from-fluent for someone who wants to go to a Japanese university.
It's quite important, then, to first try and identify what exactly is fluency to you? It might be that you just want to consume Japanese visual media (movies, tv, etc.), or you want to be able to communicate in Japanese online games.
I know what I want, how long does it take to get there?
This is still an extremely difficult question. Many people like to think "if I study for X minutes/hours a day, how long to reach goal Y?" There are simply too many variables that, if we were to oversimplify it, affect your progress towards that goal. Too many to give anyone a number without it being no better than a guess.
If we are to just focus on the literal studying, we've got questions like:
Where are you studying? What's the environment like (light, noise, temperature)? What tools do you have available (apps, books, etc.)? How acclimated are you to using those study tools? What kind of distractions? What is the pacing of your studying? (how much time, break time, etc.)
We've also got your physical/mental state:
Are you tired? Alert? Stressed? Happy to study? Pressured? Motivated? "In the zone?" How long has it been since you ate something? Are you studying other things as well? Did you just get off of work?
There's "external" factors
Is this your first foreign language? How similar is it to what you know? Do you have access to "real/live" Japanese? Japanese people? Are you in Japan? Can you practice what you learn? Do you have people giving you feedback? Are they teachers?
The list goes on and on and on. All of these have a ton of influence over how you progress.
Of course, this is not to say that anything is hopeless, just that pegging a specific time to reach a specific level of "fluency" is extremely hard to do, and I'd go so far as to say it's dangerous. If you set an unreasonable level of progress for yourself, you are more likely to get discouraged and give up.
