How many years did World War II last in Europe, from Germany's invasion of Poland to Germany's surrender?

How to get close to this number

Subtract the dates, and note the question specifies a theatre. The European war ran from September 1939 to May 1945, which is almost exactly six years — the arithmetic is trivial and the specification is the part doing the work.

The Pacific theatre continued several months longer, ending in September 1945, so a single duration for the whole war depends on which ending you use. Global conflicts do not end simultaneously, and a war described by one date is usually being described from one country's perspective.

The start date is equally perspective-dependent. Fighting in Asia began years earlier, and several countries date their own involvement from entirely different moments. "How long did the war last" is a question with half a dozen defensible answers, and this one is precise because it names both endpoints explicitly.