Okay so how many DPO is safe to test?

Jaimi • Sat, Sep 20 2014 • Jaimi. 28 years old. Herb is DH. 16 month old daughter Evelyn. Pregnant with #2. 9/23/17

This 2ww is already killing me. In 4-5DPO. My girlfriend tested at 8dpo and got a faint line. Is that normal?