I don't know about your specific situation, but I have been polyphasic
for more than a month now and I have a 10 hour chunk in which I do not
nap (for work) I do 130-430, 6-620, 440-5 and 940-10 and it works
just fine. It's really a matter of adjusting and is really flexible
once you are adjusted, especially with the everyman schedule.
Sometimes I sleep at 1, sometimes I sleep at 2, and I still feel like
a million bucks after each nap. I don't take an extra nap on the
weekends during what would be my work hours, just so I stay
consistent.
I'd say 7 hours and 30 minutes is just fine although I've never done
just two naps.