I remember the max soul level is something around 750 or so, to have 99 in each stat. I've gotten nowhere close to that though, with my highest level only being in the early hundreds, and that was nearly 80 hours on that character. I can't imagine how long it would take to legitimately get those souls if that 1.7 billion number someone else said is correct, that's just massive. You would eventually need entire playthroughs for a single level, and then it'd just get worse from there.
I remember there being a couple of really good spots to grind for souls though, with the forest covenant area being really profitable early in the game thanks to their dumb AI being so willing to jump off a cliff if you're standing in the right spot. Farming for twinkling titanite saw was pretty rewarding too where you fight Seath. An easy 8k souls for about a minutes work, and then you can throw your extra purge stones to the snake guy.