5-30 minutes.
Doesn't really seem to matter. I usually leave them 5-10 minutes but nothing set in stone. @20ml/cup it kills instantly. I've forgotten about them and left acros in the dip for 20-30 minutes without issue but I don't recommend that.
It's unreal how well it works but is gentle on coral. I've had Acros infested with red bugs and aefw that haven't showed polyp extension in months show immediate polyp extension after a Bayer dip.
Rinse it multiple times so not to get any residue in your tank. I accidentally wiped out my pod population from dipping so many corals and getting residue in my tank.