I just stumbled onto this little site that provides the following:
These audio files consist of various lengths of silence as an aid to meditation practice. A bell is rung once at the start and three times at the end.
You may download these files or listen to them in streaming audio.
It’s nice not to have to worry about the clock when sitting to meditate, and this is one possible solution. Another is to meditate “until it is done,” which is my teacher’s instruction. Kind of vague, but in the context of meditative absorption, the “arc” of spiritual energy tells you when the session has wrapped up — usually sometime between 1 and 1.5 hours.
Until the contemplative is “jhana active” (i.e., established in meditative absorption), something like a meditation timer is a boon.