This tag applies to question that deal with how long a certain event/process took.
Time is defined as "the measured or measurable period during which an action, process, or condition exists or continues, e.g. duration", or "a nonspatial continuum that is measured in terms of events which succeed one another from past through present to future" (src: Merriam-Websters dictionary).
In the context of Sci-Fi & Fantasy SE, this tag can be used for questions that deal with:
Determining how long something took
Determining what time of day some event happened
Works that deal with time as a subject (not time travel in general, but time itself - time measurement, time dilation, etc...).