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?