I didn't vote because I tend to think that the current problem with reinforcements is less a question of quantity, and more a question of quality.
It doesn't really matter whether you're attacking any enemy planet with 100 enemy ships or 1,000, you deal with them both in pretty much the same way: Send your whole fleet, retreat when necessary. Repair, then send them back in. Bonus points if they attack you so that your defenses can help clean them up.
Larger enemy forces may require a tactical missile or Spire Martyr first. If the planet has an Eye on it, you simply try to avoid alerting the horde by sending a small group to take out the guard posts, then you send your whole fleet.
In other words, regardless of what's on an enemy planet, the solution always up with: Send your whole fleet. That's rather boring, and the only real difference between 100 or 1,000 enemy units on an enemy planet is how long it takes to clear them all.
Balance using this method is tricky. The only real factor at work here is time. How much time does it take you to clear out important planets (or impending threats)? Does it give you enough time to keep playing the game, or will you get to a point where you spend all your time (as in Draco's post) attempting to defend the constantly-growing enemy threat?
This is less a question of player skill, and more a question of AI difficulty and patience.
I'd like to think we can handle AI Planet Reinforcements in a much more interesting and strategic fashion than "my horde vs. your horde" or balancing it around the time it takes to perform the task.
I think this can be accomplished in 2 ways:
1. Buff Guardians massively, reduce their spawn rates to compensate.
When is the last time you even noticed that a Zombie Guardian or group of Zombie Guardians was stealing your units? When was the last time you had to redirect a large number of your forces to deal with a Vampire Guardian because it was literally healing faster than you could kill it? When was the last time you were worried an AI Warp Gate Guardian would reach your Homeworld and spawn a massive raid?
The point I'm trying to make here is that Guardians in their current form are simply a force multiplier. Their unique, individual abilities don't really shine through that much (except in extreme cases like with the Gravity Guardian or EMP Guardian), and you deal with them all in a similar way.
Zombie Guardian - send in your whole fleet
Flak Guardian/Lightning Guardian - send in your whole fleet
Heavy Beam Guardian - send in your whole fleet
Sniper Guardian - ignore until you're close, then send in the fleet
Tractor Guardian - send in the fleet
etc.
Guardians really only act like a force multiplier, and therefore are dealt with all in a similar way. Enemy reinforcements are bolstered by these things, but they simply encourage the same kind of monotonous player behavior as the schizophrenic fleet ships spawned on an enemy planet.
What if we buffed Guardians to the point that you FEARED to activate them or ignore their presence?
Zombie Guardian - Attack it with ships that are immune to reclamation
Flak Guardian/Lightning Guardian - Attack it with heavier ships that can withstand the aoe damage
Heavy Beam Guardian - Separate your fleet into several balls, or risk taking massive losses
Sniper Guardian - Kill with long-range snipers of your own, or send a quick interception force to destroy it before you take heavy losses
Tractor Guardian - Attack it with ships immune to tractor beams or kill it before it can drag half your army away
etc.
The point is that if Guardians were a lot more powerful and interesting, you wouldn't WANT to put the entire enemy planet on alert. If you alert all the Guardians at once, you would be in serious trouble.
Which brings me to my next point:
2. Make each planet favor a certain type of Fleet Ship archetype (Fighter, Bomber, Frigate, etc.)
Instead of each AI planet having a mess of everything, populating each AI planet with a specific type of threat would make countering each planet a lot more interesting. If a planet was filled with Fighters, you could send primarily your Frigates to remove the majority of the enemy threat, then deal with the Guardians on your own terms without alerting ALL OF THEM (like you would if you sent everything).
In other words, the current problem with planet reinforcement mechanics is that to deal with the schizophrenic enemy threat of weak Guardians and random spawns, you have to send your whole fleet. Adding a greater or lesser number to the enemy reinforcements doesn't fix the problem of enemy planets being easy or non-interesting to conquer, it just makes it take longer. We're trying to put a band-aid on a bigger problem.
If dealing with the enemy force became more interesting and methodical, you would have a reason to send a certain type of fleet ship and not send everything. With this method, the amount of reinforcements on each enemy planet would be significantly lower, but dealing with each planet would take much more thought and effort from the player. I like to think these solves both problems at once.
I realize that this solution won't be very popular or well-liked, but once again, I don't think the problem lies in how many reinforcements the AI is or isn't getting, I think the problem lies in how boring and uninteresting it is to deal with said reinforcements.