The definitive answer is that there's no such thing as a maximum per se; it's simply that complications increase exponentially once you lengthen past 20% of a bone segment's length. For most people, this generally means the safe "limit" is 5-6 inches. For me, this is 6.3 inches--and for you, it might be something else. It really all depends on the length of your leg bones. Theoretically, it's totally possible for someone to lengthen more than this--it's just much, much riskier, and most surgeons won't take that chance. I know of only two people who've done: Apotheosis and this other guy who went to India. The former's results are hard to judge, since he's dishonest and elusive, while the latter still has a hard time walking (last I checked).

So, in short, measure your tibia and femur bones, then only lengthen 20% of each segment (at most).