at one sitting
During one uninterrupted period of time. Our teenage sons can finish a whole turkey at one sitting. How much research can you get done at one sitting?
at one sitting
or in one sitting
COMMON If you do something at one sitting or in one sitting, you do not stop doing it until you have finished it. He sometimes reads a whole novel at one sitting. She can go through a whole box of cookies in one sitting.
do something in/at one ˈsitting
do something in one continuous period of activity, without getting up from your chair: I read the book in one sitting.