According to the Constitution, how often is the number of congressional districts per state determined?

Answer 1

Each time there is a census congressional districs are apportioned.

Answer 2

They can be determined, analyzed, re-evaluated, redrawn... whatever you want to call it per session of Congress... means once a year; although, this rarely happens.

The Supreme Court ruled on a Texas case a couple years ago when the districts were redrawn twice within a 10 year period... said it was legit because there is no criteria except the one session rule.

Answer 3

By a census, taken every ten years, which determines the population per state. Northeastern states have lost seats in the House while the south and west have gained seats.