There are clear rules regulating that, but they may depend on the country.
Some countries require at least 6 months + 1 day to be considered a resident, others might have you have a resident even if you aren't there if "the country is the center of your family or economic interests".
What causes difficulties though is that the rules in different countries, even if clearly defined, can contradict each other on the question of tax residency. So you may well be liable to pay taxes in two (or more) countries at the same time.
You may even have to pay taxes in both countries on the same income or gains, unless there is a double taxation agreement that allows you to offset some of those taxes against each other [2]
[1] https://www.gov.uk/hmrc-internal-manuals/residence-domicile-...