If you both have negative STI tests after the window period, there would be no need for further testing.
The window period is the time between when a person comes in contact with a sexually transmitted infection (STI) and when the STI will show up on a test.
You would only need to go for another STI test if something happened like opening up the relationship, an unplanned sexual exposure or an incident that could pass something, for example one of you work in health care and had a needle stick injury.
For other people who are also reading this question it also depends on your individual situation. For example we have some clients who are in long term monogamous relationships but one of the partners has HIV so the other person comes in every so often for a HIV test. You can always speak with the person who does your testing as they would know your individual situation to let you know how often to get tested.
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For other readers, please feel free to leave a comment, or let us know if this was helpful.