On page 72 of the Basic Rules v1.0, it says:
The Order of Combat A typical combat encounter is a clash between two
sides, a flurry of weapon swings, feints, parries, footwork, and
spellcasting. The game organizes the chaos of combat into a cycle of
rounds and turns. A round represents about 6 seconds in the game
world. During a round, each participant in a battle takes a turn. The
order of turns is determined at the beginning of a combat encounter,
when everyone rolls initiative. Once everyone has taken a turn, the
fight continues to the next round if neither side has defeated the
So yes, there definitely is a rule establishing that each round is about 6 seconds.
In addition, on the next page it states:
Other Activity on Your Turn
You can communicate however you are able,
through brief utterances and gestures, as you take your turn...
As such, there is also a rule that (effectively) restricts players or PCs from speaking for more than 6 seconds, and I enforce some urgency to communications for the group I DM.
Basically, long conversations and discussions aren't permitted. If the players try to discuss what to do, but take a long time to do so, I'll point out that A) they are in the middle of combat, and so their time to speak and communicate is very limited, and B) the enemy can hear whatever they are saying to each other.
I don't use a strict real life time limit of 6 seconds, but rather focus on whether or not the character's actions are reasonable for a combat situation.
... some other players argue that this makes it
unreasonable/unrealistic for a character to speak longer than that.
To me this doesn't make sense, because in my opinion, that just
removes a lot of potential for fun and roleplaying during combat for
no good reason.
I disagree. I feel that having players remain in character helps with the immersion of a combat setting. That said, there's no reason for the DM to not allow role-play during combat, if it makes sense.
For example, a PC could try to defuse a combat situation, and all characters (PCs and enemies alike) could "pause" while the parley is occurring.
Ultimately, I've found that not allowing long discussions during combat helps keep things moving. Now, the player themselves can be given some time to think of what to do, or ask rule clarifications, etc, but I find it fun (and my players are pretty happy in general with my DMing) to not have talking be an unlimited "free action".
Games in general are in large part resource management, and I believe treating the time you have available to speak/plan/parley/etc in a combat situation is just another example of that.