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?
Best Answer
I don't have any knowledge of how long is too long for a message cantrip. However, I do not believe the DM should allow a player to send a message that is too long and truncate it for the recipient.
D&D is not themed or focused around discovering new abilities hidden inside you and learning their strengths and limitations. It's reasonable to assume that a wizard who has fireball written in his or her spellbook knows the range and blast radius of that spell, even if he or she doesn't know the exact damage in terms of polyhedral dice and hit points. Perhaps it's encoding in the arcane glyphs used to write the spell in their spellbook, perhaps the wizard has tested the spell before rushing into battle and scribbled some notes and targeting information in the margin, or perhaps it's just that wizards who cast fireballs on themselves don't tend to last very long.
On top of that, message is a cantrip, which is a spell that the caster knows like the back of his or her hand.
I find it highly unlikely that a caster would not know the limitations of their own cantrip, and if a player of mine attempted to do something outside of those limitations (ie, send a longer message), I would stop them. It's similar to how you wouldn't let a player attempt to coerce a black dragon with gold just because the player read the Monster Manual. Their character doesn't necessarily know that black dragons are greedy; that decision doesn't make sense in the context of roleplaying because the character doesn't have the knowledge needed to make that decision. With this, the problem is reversed; the character has more information than the player, and because of that increased knowledge would not make the decision their player wants them to. As a DM, I would notify the player that they're overstepping the bounds of the spell and allow them to rephrase their message or rethink what they want to do. DMs often punish metagaming because players who do metagame to the extent that it impacts everyone else's fun often have other issues that make them less fun to play with. However, with..."eso-gaming"? ("meta" means outside in Greek; esoteric means inside. I'm thinking too hard about this.) With "esogaming", a player could be sleepy, read through their character sheet too fast, didn't realize you've come up with an alternative house rule for this situation, or had a different interpretation of a vaugly-worded spell or ability (as is the case here). Correct them gently and I doubt they will make the same mistake again.