You have to fight them within the red circle on the map. If you go outside that circle the event timer will not countdown. The events vary slightly but its generally ~2 minutes per event. You don't have to kill them over and over you can just run around within the circle until the event ends. Then you may need to kill the remaining enemies.

Gotta destroy em bro

Originally posted by Crimson:

You have to fight them within the red circle on the map. If you go outside that circle the event timer will not countdown. The events vary slightly but its generally ~2 minutes per event. You don't have to kill them over and over you can just run around within the circle until the event ends. Then you may need to kill the remaining enemies.

Oh, no wonder. Alright thanks lol

If you have nothing to lose in that red zone (like its just a couple of work benches) you can just ignore it and build somewhere else in perfect safety. You can't get another raid while one is ongoing. You can't cheese that for long though because the original raid WILL despawn after 3 game days.

Raids are always one biome ahead of the current boss you've slain.
The only peculiar raid is the troll raid which will happen anytime after a troll has died in your whereabouts.
Very minor spoiler:
When you kill Bonemass you'll also unlock skeleton spawns in the meadows