A good question! How often you should visit a GUM clinic depends on your level of sexual activity and the kind of sexy you're having.
Terrence Higgins Trust has a good info section about this exact topic on its website:
"How often should you get tested? Text How often you should be checked depends on how many people you have sex with.
If you don't have a regular partner and you have casual sex you should go at least once every six months.
If you have lots of sexual partners have a check-up at least every three months.
If you get any symptoms that may be an STI (eg, sores, inflammation or discharge), go to a clinic straight away and don’t have sex until given the all-clear.
Before having sex at the start of a new relationship, have a check-up, especially if you're thinking about not using condoms (then HIV tests are strongly recommended). A sexual health screen should also include an HIV test.
If you have HIV it’s important to find out whether your viral load is undetectable (and be taking effective treatment) before considering stopping using condoms. Remember that having sex without a condom can lead to other STIs being passed on.
PrEP (pre-exposure prophylaxis) is a course of tablets taken before and after sex that protects against HIV. Consider it if you think it might be for you."