MY longest session lol would be roughly 18 hrs on the day it came out (22nd oct) i was just sittng in a big ball of [snip] it was so immense and i had been wating over a year for it a spegtacular day.Although iv'e played 22hrs on fo3 in a row but that was for plat trophy.

After about 2 hours of playing for me the loading screens become like 4 minutes long so my longest was like 4 hours on the first day

Nine hours or more.

EDIT: I assume you mean at one time.

I did a 10 hour run on day with loads of free time, but It might not totally count, because every time I pee or I got a call I left the game and then came back.

When I got it (Oct. 28), I pulled a session from 4 pm to 1 am