LaGrange,
I don't have a definitive answer, but here's what I can tell you:
Claudio Stapmi—who has probably done the most, if not the only, research on polyphasic sleep—sleeps ~6 hours at night and naps for ~15 minutes during the day. (See the bottom of page 4 of
Miles to Go Before I Sleep.) He also mentions that there are times when we are more likely or less likely to fall asleep easily. Easy to nap in the early to mid-afternoon, hard to nap between 6 and 8pm. (That matches my experience, too.)
The Goggle Group on Polyphasic Sleep chose not to allow members under the age of 18 because they were concerned about the possible health and growth effects. Because there's so little research, nobody really knows; they're just speculating based on what is known about monophasic sleep. However, the Google Group focuses mostly on polyphasic schedules, not biphasic schedules.
There are cultures that generally sleep biphasically (think Latin American siesta), which implies that sleep schedules are partly cultural.