I don't think there's a good answer. Unless you're inundated with newborn sessions, you should have a little flexibility. I now have a 2 week old and she was sleeping 23 hrs per day for a good stretch.
I think 2 days is way too short. For women with c-sections, they don't get out of the hospital with the baby for 3 days. And our hospital did afternoon discharge, so basically the 3rd day is gone. Earliest would be the 4th day. Also, for the first couple of days, the baby sometimes likes to cluster feed (feeding very frequently) which can really hamper a session.
The photographer we hired always shoots (pun intended) for 7 days past birth with a little flexibility on either side. She took her photos at Day 7 and I took mine on Day 8. I think from about Day 5 to about Day 10, the baby was the sleepiest. Pretty much anecdotal evidence for me, but the pro saying 7 days lends some credibility to that.