I think I need to put a timer on my players for their strategy sessions for every encounter. They will debate waaaay too long on how to approach every encounter. I don't mind the players working out how the characters would execute the encounter, but the players will metagame, come up with conflicting ideas (and a few weird ones), and devolve into a debate that drags out the strategy session too long.
My solution recently is to grow the encounter the longer they debated (added an additional monster to the encounter). It worked naturally for one encounter, but I would not want to artificially invoke changes all the time (which disrupts carefully balanced encounters to story flow).
I don't know that you can reasonably limit encounter planning sessions that occur prior to combat. Indeed, I don't know that you should, as it's probably an indication that your players geek out on tactical stuff.
You can do the following:
a) Charge them in game time for the time that they spend planning. So if they spend 30 minutes planning, and the enemy is in any fashion aware they are under threat, they'll use that time to shift positions, fortify, and prepare within the limits of their intelligence.
b) Insist that any planning and communication that occurs during combat follow the rules and the fiction of the game. So there is no time out in the middle of a melee to figure out what to do or to negotiate back and forth how you think other players should behave. Players communicate with each other only on their own turn, they can communicate only a single sentence, and they have 6 seconds or so to specify their plan of action or they forfeit their spot in the turn order. Monsters can overhear their planning and react.
c) Have monsters that are proactive and reactive. For example, it would not be out of character for a smallish goblin tribe to pack up and move if given an hour or two to recover from a hit and run attack. If the players planned a dramatic attack, they'd swoop down onto an empty cave with all treasure removed. That's IME always funny, because it shocks the players completely the first time it happens. Meanwhile, the tribe would be making best speed for the nearest allied tribe. It likewise wouldn't be out of character for a largish goblin tribe, if targeted by a hit and run raid, to send for reinforcements from its outlying fiefs, and to immediately send out scouts to try to figure out where the PC's were holed up, to try to harass the PC's at night to prevent them from sleeping, and to generally make themselves a nuisance rather than wait for the PC's to do something.
d) Keep them on their toes. Planning sessions at length require good intelligence gathering. Generally speaking, it's almost impossible to gather perfect intelligence on a peer level foe who knows what sort of tricks an enemy will pull to gather intelligence and will work to counter that. Without perfect intelligence, extensive planning is likely to go horribly awry on the basis of missed details and false assumptions. If the PC's are going heavily into intelligence gathering and tactical planning, it's a sure sign you've not been putting enough thought into enemy counter-measures because surely the Pc's aren't the first people in your creation that have ever tried this stuff.