Along is a measure of time.
When you install new software or upgrade existing software on your computer an along is the interval of time from the instant the status notification on the screen changes to “about one minute remaining” until the installation or upgrade finishes.
Note: There is some debate as to the starting point of an along. Some experts claim that it begins when you first notice the “about one minute remaining” message, not necessarily when it first appears. Others claim that it begins when the message first appears, whether or not anyone notices the message the instant it first appears. Here at The Words Project, we prefer and use the abstract definition, i.e. a period that begins when the message first appears, regardless of whether anyone is around to see it. (We also believe that the oft-sited falling tree in the forest does make a sound.)
Typically, an along lasts anywhere from 15 seconds to two hours, but it’s sometimes infinite.