There is no reliable way to detect this. There is no cast or anything that can be tracked. The only way is to detect when anyone got damaged by it to schedule a timer for the next one.
Even then the transitions for faster casts are estimated too. Sometimes he does it 1-2 casts earlier or later.

I think it would work decently but idk if I want to invest the time into it. Also this would not work if everyone dodges the telegraph but you can probably count on 1-2 people always getting hit.