In Macbeth, what does Malcolm mean when he says, "The night is long which never finds the day"?
Macbeth's reign is a reign of darkness. Darkness, like chaos, can occur, but eventually light, order, and the proper king must follow the darkness just as day eventually follows even the longest night.
Even though Macbeth's rule is so horific that it is like being plunged into eternal darkness, Macbeth is mortal and so, therefore, is his time. The day/ night imagery, along with the reference to Macbeth as a tree that is "ripe for shaking" suggests that everything fits within a natural cycle or order, and that change is inevitable. Also, that the time for that change is at hand.
The reference also alludes to both Macduff and Malcolm's emotional state. Both characters have wrestled with depression in this scene. At the beginning of this scene, Malcolm was ready to sit under a tree (in some "desolate shade"!) and cry until there was nothing left to cry. In contrast, Macduff's response to grief is to feel it deeply, and then take immediate revenge. Macduff's response may have also helped Malcolm pull himself out of his own grief.
From this perspective, the statement seems to signal a move in Malcolm away from his own emotional and spiritual darkness.
The actual line Malcolm says is "Receive what cheer you may/ The night is long that never finds the day." It is perhaps noteworthy that when Malcolm speaks these words to Macduff it is after they have agreed to confront Macbeth together: their power is ready, Macbeth is ripe for shaking and the Powers above are armed. It would be a long night, indeed, if Macbeth were to continue his reign and never meet the day. In this case, the day that Macbeth meets must be the "instruments of the light", not the least of which are Malcolm and Macduff (and the Powers above who are of course on the side of the light and the right).
As with many lines in this play, the meaning is doubled.
One meaning is a literal meaning describing an emotional sensation. By that I mean, a night that never finds day would be a night that never ends. The darkness would go on forever, and sometimes we all feel like that, when the world is crashing around us.
However, the second meaning is also literal: you'd have to wait a long time to have a night that never finds a day, because that never happens. Morning always comes; things always get better.