The beru is an Akkadian unit used in Mesopotamian texts to measure distance. To find the length of the beru, we must first look in the Chicago Assyrian Dictionary (warning: large pdf file), page 208. The Dictionary records a large number of references to this unit of measurement, few of them being specific enough as to allow for the measurement of its length.
In 671 BC, Esarhaddon marched to Aphek at the border of Samaria and further marched 30 beru of land to Raphia (full account here, pg. 142). The distance from Aphek to Raphia is about 72 miles, making the beru to be about 2.4 miles. Some authorities wish Esarhaddon’s Aphek to be that at the border of Megiddo, rather than the border of Samaria, however, the relevant text expressly contradicts this assertion. Secondly, Ashurbanipal records marching for 6 beru from Damascus to Hulhuliti. If Hulhuliti is to be identified with the present Halhaleh, the 6 beru would have to be covered over a distance of 34 miles, making the beru be some 5 2/3 miles in length. Thirdly, a text of Nebuchadnezzar I records that the distance between Der and the Sha’ur was also 30 beru, the journey from Der to the Sha’ur being some 160 miles or more, making the beru be some 5 1/3 miles in length or more. Fourthly, as mentioned in the CAD, on his campaign to Bazu, Esarhaddon mentions he covered four beru in two days, making sure the beru was not a measurement directly varying with the number of days traveled. Thus, the beru was a somewhat inconsistent measurement of distance.
The beru, however, was also a unit of time; the double-hour, as also mentioned in the Chicago Assyrian Dictionary. Due to the significant variations in the length of the beru among Mesopotamian texts, even those only a few decades apart, it makes sense to consider that, as the beru of distance was likely based on the beru of time, the variations between the distances of the various measurements of the beru could be explained on the basis of marching speeds. Esarhaddon, for example, moved very slowly while moving to Raphia, frequently stopping for taking on well-water, while Ashurbanipal apparently wanted a surprise attack against the Arabs temporarily residing at Hulhuliti.
These conflicting data indicate the beru was a unit of distance without a great amount of consistency of measurement to it which was probably based on the unit of time of the same name.