In D&D 5e, many spells have a fixed duration measured in common units of time – minutes, hours, days and so on. Does the caster of a spell have any special insight as to when a spell they cast will end, and if so, is this knowledge vague ("about half", "very nearly gone") or precise ("in five, four, three, two…")?
This question concerns all spells with a duration, regardless of whether they require concentration. Whether a caster is aware that a spell has ended, other than due to its duration having expired, is outside the scope of the question.
Best Answer
They probably don't get any special knowledge
But the caster does have the knowledge of how long the spell lasts and when it was cast. Depending on how the spell was cast and who was present, the spell’s duration may in fact be only known to the caster.
Two possibilities that may allow a caster to keep track of the time left in a spell.
The Keen Mind feat (PHB, p. 167) states:
Being able to track time with uncanny precision would likely allow for the caster to know the duration the majority of the time.
A pocket watch