Article II Section 3 Clause 1. This could be once per congressional term, once per presidential term, or once per month.
I believe that this also implies strongly that Congress must accept the SOTU, whether by reading it or being in the House to hear the speech. These congresscritters who deem themselves too good to attend are in malfeasance of duty.
There are very few enumerated duties of a Congress member. Since they don’t want to do a proper budget, the least they can do sit for a hour and listen to a speech.