How long, cumulative, can a US citizen stay in England over any given year? How is the year measured-calendar, from the time of first entry, etc?
In general answer is that you can stay for up to six months without getting a visa. I'm not sure what their rules are about cumulative time but the time doesn't matter for the calendar year, I don't think. So you couldn't come for the last 6 months of one year and the first 6 of the next because that would be a whole year. There are also many pointed warnings that if you come to the country with an exit date 6 months away you will effectively have to prove you have enough money to support yourself during that time. And if you can't - to their satisfaction - they will turn you away at the border and send you back home. I think the bottom line is that if you think you have need more than 6 months out of the year look into the process of applying for a visa. http://www.ukba.homeoffice.gov.uk/countries/usa/?langname=UK%20English Hope that helps,
http://www.ukba.homeoffice.gov.uk/visas-immigration/visiting/ "If you come to the UK as a visitor, you are normally allowed to stay here for a maximum of 6 months." Sorry, it has no information on how the length of time is measured. I know in the Schengen zone it starts with the day of arrival. There, also, one can leave the Schengen zone and it does not count against the 90 day limit for visiting there. It is, however, best to keep documents of departure (a ticket of exit and re-entrance). The US Dept of State (responsible for passports, etc) doesn't have specific information on UK immigration laws specific to US citizens, they refer to the above noted website.
I'd suggest you find the closest UK consulate (or such) and ask them the question, if you don't get a good (more specific) reply here.
Six months is the maximum period on one entry as a general visitor and cannot be extended. There is no formal equivalent to the Schengen '90 days in 180 days' concept. However, the following appears in the immigration rules that the immigration officer at the border has to consider at each entry attempt whether an individual: "Does not intend to live for extended periods in the United Kingdom through frequent or successive visits" This is often interpreted as more than any six months out of twelve needs some specific justification, but does not mean necessarily refusal.