How often do you need to be screened for std's?

1 years. Everyone should be checked for stds every year if you are sexually active regardless of marital status.
It depends. If you are in a monogamous relationship, get tested yearly. If you are single and not in a commited relationship, get tested every 3 months. Even better, bring your partner so you can both be tested at the same time. Honesty will bring you closer together.