Here's an interesting thread for you guys:
http://www.reefland.com/forum/reef-a...ompatible.html
The planktonic stage length may vary with the temperature, but all the species that I can find info on show a length of several weeks to a couple of months. I found a hint that there may be a related species or two with a shorter planktonic period, but I couldn't access the articles. I'll see if I can find some specific info for you guys.
As an example, Nassarius pauperatus is known to have a planktonic stage of at least 21 days at 26 degrees C.
Cheers,
Don