Yeah, it depends on your average cycle length. I ovulate very late in my cycle, so I don't start using them until CD 20. But if you ovulate earlier than that, start around CD 10 so you can get the hang of them. I also recommend testing twice a day, limiting your fluid intake, and writing the date and time on the stick so you can compare them after the fact. Some women see a fade in/fade out pattern, so keeping the sticks to compare them is helpful.
Good luck!