I am creating an encounter for a game I am DMing and am wondering how you calculate CR for a running battle. What I mean by this is that characters will be fleeing through a zombie infested town, with zombies popping up at regular intervals(every 2-3 rounds).
Theoretically, the battle would never stop, so it's effectively one encounter, but I don't think standard CR calculations would work. 15 CR 1 monsters at once is a lot harder than 15 CR 1 monsters in groups of 1-2 in a steady stream.
Any guidelines on how to calculate an appropriate CR in order to balance this?