GMs who ran a fantasy game with a necromancer player, how did you handle zombie accumulation?
From my reading of the Basic Set and Magic (and even Zombies!), it seems like there is no clear rule for how long a zombie lasts nor to limit the number of zombies a caster can control (for instanelce, in D&D the spell lasts 24 hours and allows 4 controlled zombies at once). All I see is a vague sentence in the Basic Set about zombies decaying into skeletons. And Zombies! also doesnt explicitly deal with this, it seems more geared towards running campaigns that focus on zombies as a setting fixture.
I have a player who is casting Zombie on pretty much any corpse they can get their hands on and I dont really know how to keep them from just amassing a game-breakingly-large zombie horde. I dont want to get in the way of the player enjoying a necromancer, but I also don't want a horde of zombies to take over play from the rest of the party, either, not to mention balance issues and the plot line. The game is a run-of-the-mill medieval fantasy campaign with a party of 5 adventurers.
I would love some advice about how to handle this. Or am I just missing something in the rules?
Edit: Thank you all for the advice and information, it has helped me to get my head around this topic much better.