The timing is somewhat unique to each layer in part because each layer’s floors and the waves that compose them are a bit different. I only remember layer 8 because it took me a lot of attempts; a lot of the first few floors involve mostly elites and rare elites, but basically on floor 1 what would happen is two gates would spawn elites and when both elites were brought below 50% or one was killed, the other two gates would spawn one.
You really just kinda have to experiment if you think the timing is important on that.