Well, taking a straw poll, the website of the
French embassy in London says: "Short stay Schengen visas are visas that allow the holder to travel freely within the Schengen space, including France, and spend a maximum of
90 days over 6 months. Those are issued for various purposes such as tourism, shopping, business or family visits."
The website of the
Swedish embassy in London agrees: "Visas are granted for a maximum of
90 days at a time."
On the other hand, the website of the
European Commission talks rather vaguely of three months: "As a citizen of a third country you may enter and travel within the territory of the Member States applying in full the Schengen provisions for a period of up to
three months, provided you fulfil the entry conditions..."
In most cases the difference is immaterial, but for some people in some cases it could make a difference, as you say, sakura.
Having said that, are not people admitted to the UK for six months? That presumably means six calendar months, and so the time could vary depending on the date of entry. But that seems pretty definite: there doesn't seem to be any doubt that it
is six months.