Should happen about 4 to 5 days after missed period or when after implantation because once implantation happens then your hcg levels increase which is what it needed for a positive test.
Good luck!
In order to knwo when you can test with blood work, you need to know either the first day of your LMP or when you ovulated.
Implantation can occur anywhere from 6-12 DPO and you can do blood work 2-3 days past implantation.
Hope that helps you out. Good luck to you!