This question is generally not something we consider on-topic, but I'll give you my honest advise anyway.
The Short Answer:
A long time. A really really long time. Your mileage may vary, but expect it to take several years.
The Long Answer:
"A journey of a thousand miles begins with a single step"
I would say in my own opinion that you're going to want near-fluent levels to understand games. If you memorize 100 words from one game, that's all well and good but you may never see any of those 100 in any other given game. A hurdle in your way is that there isn't any program to learn slangy, video-game type language to the point of fluency without also having to pick up standard Japanese (and I would not recommend trying to do so).
Any program telling you you can "Learn Japanese Fast!" isn't telling you the whole truth. Programs like Rosetta Stone will only help you so far. My personal (limited) experience with Rosetta Stone are that you'll learn some vocabulary and be able to recognize words and sentences, but you won't be able, say, understand a TV show.
The good news is that you want to learn Japanese. But! Video games are fun, and understanding video games will feel fantastic. Learning Japanese to fluency will take a lot of hard work and effort, and you're not going to be able to pop in a brand new Japanese video game and understand it perfectly for years. It's really hard to stay dedicated to something without that instant gratification, and that's the same for everyone. You have to keep at it, because it takes time.
It's good that you're willing to spend money on materials. I'm a pretty frugal person so I thought for the longest time I would not spend money to learn Japanese as there are plenty of free resources on-line. I ended up changing my mind on that point because when you buy materials you're paying money for something that someone was paid to make so that you can learn. You're paying for their quality (and compensated) work. Free and open information that people want to share with the world is absolutely amazing and there's no reason not to use it as a tool, but paying for textbooks/workbooks etc is a good base for your studies. (As an added bonus, actually spending money on something makes me personally want to actually use it to get my money's worth.)
A third group of learning materials is native sources (newspapers, TV shows/movies, video games, books/comics, etc). Get and use these sources as much as you can handle and more. For me, this is where what I was talking about instant gratification comes in to play. When I do flashcards, that's all find and dandy, and I "sure, I learned that word enough to get the flashcard right", but when I hear/see a word in a drama or book, I think "Yes! I know that word! I'm so smart!" (or sometimes just seeing the word in context makes remembering it so much easier the next time it comes around on the flashcard program). There's also a bizarre phenomenon where you're likely to come across by happenstance a new word you just learned in a completely unrelated context that day (I'm sure it has a name). Those words get jammed in your memory, but if you don't have Japanese coming at you regularly, the chances that you'll see those kinds of words diminishes.
All in all, it's going to take months and months and years and years to achieve near-native fluency. Maybe less if you live in Japan or don't have a job. The tricky part is learning a language isn't about just one aspect; you can't just memorize a stack of kanji and be done. One day work on this aspect, the next, that aspect. Look at your weaknesses and attack those points. Don't worry if it seems like you're at a brick wall, or worse, a plateau, because this too shall pass as long as you keep at it.
Some final points of advice:
Do a little every day (within reason...sometime real life happens and that's okay).
Don't judge your success or lack thereof based on what others have accomplished or failed to do.
Focus on why you want to learn Japanese, not how cool it would be to be able to speak it.
"A year from now you may wish you had started today"