It's kinda like asking "how long is a piece of string?". There's no single answer. First, is it whole or pellets? Second, what temp. Third, what's the fermenter geometry? All will have an effect to some extent. For instance I fine that pellets in my conical will usually drop in 2-3 days at 35F. IIRC, whole hops in a carboy or bucket took longer, but its been a long time. Always remember, the beer makes its own schedule.