From personal experience, anything besides full 7.5 hours or 6 hours plus 30 min nap, is unsustainable over extended periods of time. Preferably 7.5 hours + 30min nap to stay energetic throughout the whole day.
Your needs may be different of course, but I'd be very sceptical about not getting several complete sleep cycles every night.
Your needs may be different of course, but I'd be very sceptical about not getting several complete sleep cycles every night.