I told my kids as soon as I knew.
I know how terrible it feels to know that something was going on but that no one values me enough to fill me in. It's a double whammy - the curiosity and exclusion as well as the message it sends.
Will the impact be less if you wait? I don't think so, and there is time for resentment to grow.
You know your daughter best, of course. If she's the sort of person who wants to coasts through life without seeing nasty stuff until she has to, by all means, wait. If she's the sort of person who likes to be involved in whatever is going on, she may be very hurt if you keep it from her.
Don't know if this all helps, but hopefully you'll find something useful here.