We have very little detail about the length of time that the lár was in use. It was explicitly associated with the Númenóreans, and so must have developed during the Second Age. Our only specific historical reference, however, is in the description of the march of Isildur from Gondor toward Arnor at the beginning of the Third Age. How long its use persisted into that Age is entirely unknown, and indeed it might still have been in use even into the Fourth Age.

The lár was defined as a distance of 5,000 rangar or strides, a total distance that Tolkien calculates as 5,277 yards, 2 feet and 4 inches (or exactly 4,826 metres). By comparison, a league (defined for this purpose as three miles) represents a distance of 5,280 yards (or slightly more than 4,828 metres). The difference between a lár and a league is thus a matter of six feet and eight inches (or a little over two metres) - a difference so trivial that, for practical purposes, the terms can be used interchangeably.

It should be noted that Tolkien was careful to qualify these calculations, because the exact length of a lár could not be measured directly, and had to be inferred from historical evidence. In addition, the definition used here (and by Tolkien himself) of three miles to a league is not universal, and different definitions of 'league' exist. Nonetheless, it was clearly Tolkien's intention that a lár should be taken as effectively equivalent to a league.