I basically just cut the flower cluster off when most of them turn brown. A pair of new ones will grow from the next node -- you'll probably see them either already growing or starting o grow.
Sometimes, I'm trying to cut a dead flowerhead off and bees, butterflies, or hummingbirds will come and start sipping from the pitiful few flowers left on it, forcing me to back off and wait for them to finish and go tend to more fresh and fully open flower clusters. Hummingbirds will even scold me for reaching for THEIR flowers. (I always picture myself at a restaurant stopping a server from taking away a plate: "Oh, wait! I'm still working on that....")