It is really not a matter of how much time you need, but rather what you choose to spend that time on and how :) Do you want to be fluent in general or in specific topics?

For the latter, it would be enough for you to study between half an hour a day and 2 hours a day max to become fluent in any topic after as little time as a month or two, as long as you learn that area's specialized vocabulary. Please note that even Russian speakers themselves are not fluent in many topics - most of them would certainly not be able to sustain a deep conversation about engineering or medicine or useful forest plants for more than a minute :D

If general fluency is your goal, focus on your personal dictionary - what are the topics that you find yourself discussing with others most often? Is it work, studying, language learning, religion or maybe some hobbies? Then identify the words that you use frequently, look for their translation, compose some example sentences and then find a native to practice them with, and practice them often :)

If you apply this strategy in a very narrow, focused manner (rather than just going along with whatever vocabulary that the textbooks are offering you), you will notice that you don't need 5 or 10 years to become fluent. In fact, you can become fluent in the topics that matter to you personally in as little as 5-6 months, and then keep working on the rest of the vocabulary as you go. You will also see that your enthusiasm level will increase by a lot, since you will be able to communicate with natives about topics that are important for you, and this rewarding experience will boost your motivation to keep going further :)

Wish you good luck!

Practice makes perfect. About 10 years of it, I guess…

But you don't have to aim for a native level. Each level up is really time consuming (the same works for English, I guess: being a fluent/native speaker is 10 to 100 times harder, than being intermediate).

Also, native Russian learners (kids) make mistakes, which are similar to ones people from other countries make when they learn Russian.

Hi! I think it depends on your goal.

If you want to speak informally, like usual Russians do in everyday life then it's better to listen to Russian speech as much time as you can and try talking to native speakers whenever you get the chance. The most common words and expressions will stand out and you'll learn which to use in which situation without trying to memorize too hard. If it's possible, ask the native speakers you talk to to speak slower. 

If your goal is more formal speech or literary language then reading simple texts out loud would help a lot, even if you won't understand every single word. 

It only sounds intimidating!