It depends on people. People who are 7 head tall usually have their leg about 3 head tall. I didn't master anatomy yet, but I'd recommend looking at references and measure people's leg proportion and then you might get a general idea.
Thanks a lot for explaining this my friend! I have finished PART 4 of this tutorial, thankfully i got one extra week of vacations in my university! So i have more time to practice! I am gonna practice some body drawing in order to be in perfect shape to start tutorial 5!