It was EXPECTED to be a part-time body. That’s why Article 1 stipulates a minimum number of days that they must convene. The expectation was that it would be difficult to get them to show up in the first place.
(The preceding words in parentheses were superseded by the 20th Amendment, section 2.)
2. The Congress shall assemble at least once in every year, and such meeting shall begin at noon on the 3d day of January, unless they shall by law appoint a different day.
Where does it specify the minimum length of session??
Article I stipulates that they meet annually. It does not stipulate the number of days. In fact Congress sets its own schedule. Perry can say that they will only meet 5 months out of the year, for example, and Congress can tell him to go pound sand and meet for 12.