At least once a month EOM
Do any providers you know of get regularly tested for STDs?
The ones I know who get tested regularly normally use public health clinics or go to Planned Parenthood.
Many will either show you their results when you meet or they show you the results as one of the pictures in their ad online.
The way I see it, if you have encounters with providers, the number of partners and the math kinda voids the test about 30 seconds after it is given. The safest thing anyone can do is just use condoms for any and all sexual contact. Too many folks have mental blocks about condoms but it will save your life in the long run.