How many years did the Cold War last, from the end of World War II to the collapse of the Soviet Union?

How to get close to this number

Subtract the dates, and note that both endpoints are conventions rather than events. The period is generally dated from the late 1940s to 1991, which gives a span in the mid-forties. Unlike a war with a declaration and a surrender, the boundaries here were chosen by historians afterwards.

The defining feature in the explanation is what makes the duration so long: it never became direct military conflict. A confrontation that cannot be resolved by fighting has no natural end point, so it persists until one side's political system stops functioning — which is exactly what happened.

The proxy conflicts are where the duration becomes misleading. It shaped nearly every geopolitical event of the century, from major wars in Asia to the space race, and those were not cold at all for the people in them. A term describing the relationship between two powers obscures what that relationship cost everywhere else.