There is not really a hard and fast rule for this, it's a matter of table rules for the most part. Table rules are simply the 'soft' rules that you, the GM, set for your players while they are playing at your table. Different GMs handle it different ways depending on their preferences and their player's preferences.
Per the Rules as Written, this means that could talk to each other or to an NPC as a Free Action. For example, demanding that the horribly outmatched goblin surrender would be a Free Action. Alternately, the Fighter could yell to the Cleric "I need some healing here!"
But, Out of character Table-talk is the domain of table rules. If you do not wish for your players to be able to discuss tactics out-of-character, then inform them of such. And if they start talking tactics, just assume that they are actually talking out loud in-character and their enemies can hear them. But if they are newer players, you might want to let them discuss what they want to do...at least until they get a good grasp of the game and the system.
Additionally, how much you can talk as a Free Action sits again as a table rule. If you want to play fairly strictly to the 'spirit' of the game, then a single round is six seconds long. So if they are going to talk as a Free Action, it needs to be less than 6 seconds long...no lengthy debates allowed. But, again...this is not specifically laid out in the Rules As Written, and is up to your discretion as the GM.
So, along with dice rules and the like, the degree to which you let your players metagame and table talk with regards to strategy, and how long you let them talk as a Free Action is entirely up to you, as the GM.