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?
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.
Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. (PHB 201)
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.
Per the spell, there are no limitations to what you can say. This is contrasted by other spells, like Sending, that have a strict limit on what you can say. But, even though Message doesn't have a limit, you are still limited by the other game rules. Just because you are not limited by the spell itself (other than the 1 round duration) does not mean you can break other rules.
What do the rules say?
Page 181 of the PHB describes a round as lasting 6 seconds. On page 190, it goes on to explain (under "Other Activity on Your Turn")
You can communicate however you are able, through brief utterances and gestures, as you take your turn...
The DM might require you to use an action for any of these activities when it needs special care....
What this means is that there is a precedence for communication being limited during combat. The former snippet of the quote is for doing things while communicating. What you are doing with Message is dedicating your entire action to speaking and casting, so you are not limited to "brief utterances", but you are limited by the concept of time in a round. If you can't say it in 6 seconds, it should be cut off.
Normally speaking, if you said the same message aloud as not part of the spell, it might also require an action to get it out (DM's discretion), since you are giving special attention to communication and not spell casting or attack rolls. However, with Message you are whispering the message as part of the spell, so it requires no extra time or effort.
Your message should be limited by the 6 seconds, no more or less. There is no precedence for a spell effect and a casting time being exclusively part of the round without any sort of indication that it is doing so: you use your action to cast and whisper the message. Not cast the spell then whisper.
Given the spell's text and its status as a Transmutation spell, the initial message should be considered part of, if not entirely, the verbal component of its casting, and the duration of the spell used for a potential reply.
Effectively, Message is a spell that transmutes another player such that they can hear and return your message. Given that, one should hold the player or NPC on which the spell is cast, not the player casting the spell, to the six-second limit, after which the remainder of the message should become normally audible.
