This is very much a "How long is a piece of string?" question, and there are many many many many different opinions.
I'd say the most common is high krausen (24-36 hours) + 3-5 days prior to packaging.
Another common opinion is that a single large dry hop is fine. I don't have the time/resources to do both and double-blind triangle test them, but I'm happy with the results from just doing a single large dry hop.