Multiple visits to the UK

Last year I made multiple visits to the UK from USA to visit my girlfriend there. My stats never exceeded 3 weeks each and cumulatively for 2017 it was approximately 4 months.
I am enrolled in Registered Traveler program with UK border allowing me to just scan my passport to enter without ever having to see an agent.
This year... my stays will be longer. Perhaps up to 2 months each time. Does the 6 month rule (without needing a visa) apply to consecutive months or 6 months total for the calendar year of multiple stays involved?
Best Answer
The only hard limit is that each time you enter you will be granted "leave to enter" for 6 months by default. This leave to enter automatically lapses when you leave the Common Travel Area (i.e. UK and Ireland), and at your next entry you get a new leave to enter.
However, just because it is the only hard limit doesn't mean it's the only limit.
Each time you enter you also need to convince the Immigration Officer that you're not attempting to (among other things) "live in the UK through frequent or successive visits". There are no hard rules for what this means, but if you're spending more time within the UK than without it, this behavior will raise so many eyebrows that you will find it progressively harder to convince the IO that you're a genuine visitor -- and sooner or later you will fail and be refused entry.
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Can I visit the UK multiple times?
Although there is no specific maximum period of time which you can spend in the UK in any period, as long as each visit does not exceed 6 months, your application for a Standard Visitor Visa will be refused if a caseworker has reason to believe that you are living in the UK through frequent or successive visits.How often can you visit the UK as a tourist?
You can visit the UK as a Standard Visitor for tourism, business, study (courses up to 6 months) and other permitted activities. You can usually stay in the UK for up to 6 months. You might be able to apply to stay for longer in certain circumstances, for example to get medical treatment.Can I enter multiple times on a UK visitor visa?
If you need a visa, you must apply for a Standard Visitor visa before you arrive in the UK. Your visa will be multiple entry, so you can leave during breaks and return to the UK using the same visa as long as it is valid.Is the UK 180 day rule per visit or per year?
Visit visas allow stays of no longer than 6 months (or 180 days) per visit. Visit visas can be valid for long term periods (1 year or more), facilitating multiple visits. Additionally, those using visit visas are in general more limited in the activities they are permitted to undertake while in the UK.UK MULTIPLE ENTRY VISIT VISA - TOP TIPS |UK VISA|UK IMMIGRATION|UKVI|UKBA|2019 HD
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
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