Upvote:8
Non-visa nationals who arrive in the UK for business or tourism will by default get leave to enter for 6 months, unless the immigration officer who processes them finds something amiss with their explanation.
Such a leave to enter automatically expires when you leave the UK. This is stated in Paragraph 20A of the rules...
Leave to enter or remain in the United Kingdom will usually lapse on the holder going to a country or territory outside the common travel area.
The next time you apply to enter, you can get a new leave to enter for 6 months, if you still don't trigger the IO's suspicions.
The precise number of days you have used of your previous leave to enter is not important. What is important is that you don't give the IO the impression that you're trying to live primiarily in the UK and just exiting temporarily in order to get a fresh leave to enter. If he gets that impression, you're likely to be refused outright, or be given a very short leave to enter so you can bring your affairs in order and then leave voluntarily -- it's never "6 months minus what you have already used up".
(Also important is whether the time you stayed in the UK matched what you said when you entered, even if you left within the time your LtE was valid for. If you changed your plans dramatically and can't explain that satisfactorily afterwards, your credibility will be hurt. We have seen cases of travelers who were refused entry for this reason).
Note in passing that the leave to enter you get at the border is not a visa. A visa is something you apply for in advance.