How long after birth does hcg leave your system?

My son is 6 weeks old, had sex at 4 weeks postpartum, I had ewcm the next day so I was going to just check and get a couple dollar tests, but I don't know how long hcg from pregnancy stays in your urine? Of its positive I won't freak out since it could still be left over, if it's negative and I test in a couple days and it's positive then I will freak out