2 or 3 years plus.
I started out wanting to make it to six weeks, because I heard if you made it that far you were good to go (as far as wanting to continue). I really wasn't too educated then, I mostly just wanted to do it b/c DH said I couldn't.
Still going strong at 5months, and not planning on starting solids until at least 8or so months... or weaning... ever...