Most practitioners consider 10% quite safe. After 10%, the risks increase. Whether you are susceptible to those risks and/or how much you should lengthen involves many conversations between you and your surgeon and much "waiting and seeing" to see how things go during the process.
I lengthened almost 25% with no adverse effects; however, I did not do so lightly. Doing so involved a millimeter by millimeter assessment. Notwithstanding the above, most people seem to be able to get 15% if they want it and are willing to work for it.
To roughly measure your femoral length, sit in a chair. Press against you hip to locate your femoral head (a ball of bone that usually hurts if you lay on your side on a hard surface). Put one end of a tape measure at the top of your femoral head.
Measure down the side of your leg to the bottom of your femur. Unless you are really fat, you should be able to feel a rounded bone end on the side of you leg right about where you can see the top of your kneecap. Calculate 10% of that length for safe lengthening.