1
general::
Phrase(s): at a sitting
at one time; during one period. (Usually refers to an activity that takes place while a person is seated.) • The restaurant could feed only sixty people at a sitting. • I can read about three hundred pages at a sitting.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
At one time, during one period. For example, The cruise ship could feed about 500 passengers at a sitting, or We read the entire poem at a sitting. Since the word sitting implies just that posture, the term means "during a period when one is seated and engaged in a single continuous activity."
American Heritage Idioms