Leg hair never stops growing. It falls out after it has reached a certain length. People think it stops growing because old hairs are often being replaced by new hairs.
To further understand this condition, you need to learn the growing process of hair first.
Hairs start growing from hair roots, or follicle, located underneath the human skin. The hairs get their foods from blood vessels located at the top of your hair follicle. These blood vessels supply the hairs the much needed nourishment for growth. Each of your leg hairs goes through four distinct stages between growing out and falling off. These four stages are: catagen, telogen, anagen and exogen. Each hair is going through one of these four stages of the hair growth circle.
The length of the anagen phase of your hair follicle is solely responsible for your hair length. What determines how long this stage lasts is genetics, but your hormones and stress levels can also play a role. Obviously, the anagen period of your arm and leg hairs are quite different from that of your the hairs on your head, which explains why your leg hair can't grow two feet long without trimming.
About 85%-90% of your hairs are in the anagen phase at any given time, 1-2% of your hair are often in the catagen, and about 10-14% of your hairs are always in the telogen phase. However, high stress levels and very extreme stress cases have been identified as a possible factor that can trigger a premature stop in the anagen phase in an individual. This can make your hair growth progress into the telogen phase, even up to 70% of all the hairs in your body. When this occurs, most of the hairs that are still in their growth stages in your body can all fall out at once.