First off, yes I'm bragging. But with the honest intent of sharing what I feel is opportunity with you to do at least what I've done with making isk.

I've been playing for close to a year and a half to two years now, for an average of four to five hours a day, and I believe there is only one answer to your question. Make as much isk as you can and want to, because there is no upper limit. Much like in real life, you ability to earn is constrained only by your imagination, creativity, and effort and even less by chance than in real life (real life is much, much harder). I've made probably around 6 or 7 billion, including all my assets. In liquid isk, I've got between 4 and 5 billion at any given moment and all the best ships, mods, and ammo I could ever want or need in multiple locations throughout New Eden.

There are so many ways to make isk. The best I've seen was from hands-on trade (but that can be a real grind), multi-region market manipulation (it's a part of most any trade I do), and nullsec anomaly ratting, the last of which is my preference. If you've built up your trade skills adequately, you can also do afk, long-term trading which yields steady profit over time, while you focus on ratting profit.

As a null-sec ratter with level 5 upgrades in-system, I easily earn an average of about 250 million/week to significantly more depending on how many Macharial BP drops we get. Every so often, I hop down to high sec and reinvest some cash into long-term trade which earns me passive money in the background, but I've never bothered to track it, so long as I'm clearly profiting. And I a make sure that I am. I would estimate I make between 5-10% on my investments, sometimes more, but, again, it's passive, which is awesome. Also, 5% of a billion is 50 million isk, though that's over a month or so. I invest a lot of cash to make a lot back without working for it.

There are very creative ways to make isk. Look at Chribba, Somer, the Mineral Exchange, Eve Online Hold'em, etc. Some of these are riskier investments but have higher yields. The point is, do whatever's the most fun for you, but you don't have to be poor while doing it. It just takes enough discipline to split your time between earning isk doing something slightly less fun and doing what you really like. But the secret is, the more isk you earn, the closer together those two things become. Or at least that's been my experience.