I hit 100 billion tonight It took almost a year to do it but I play pretty casually. As long as I'm making enough to plex all my accounts plus another 10 billion or so extra I'm satisfied. Lately it's only been taking 4 or 5 days (at 1-2 hours a day) to make that.
I figure if I spent 2 hours a day every day I'd make over 40 billion a month and that number would keep going up as I have more and more to invest every month until I hit the saturation point. Lately though, I haven't been spending much more than about half an hour at a time working the market.
Really the problem is it's so easy making ISK I'm getting bored with it. On the other hand the whole game is boring, I probably would have quit playing a few months back if it wasn't basically free to play because making ISK is so easy.