Does the Message spell truncate messages over 6 seconds long?

The Message spell’s duration is “1 round” (PHB, p. 259) and a round “represents about 6 seconds in the game world.” (PHB, p. 189)

Is a message longer than “about 6 seconds” truncated? Does the caster know it’s been truncated? Does the spell accept the too-long message anyway, or is the caster forced to limit their utterance to 6 seconds to start with?