Is it an article of faith that Catholics must believe, or are we free to believe that there is somehow an end to their suffering?
The teaching that the punishment of Hell lasts for all eternity is a dogma of the Catholic faith. Catholics must believe it by faith.
Excerpt from Fundamentals of Catholic Dogma by Dr. Ludwig Ott:
The punishment of Hell lasts for all eternity. (De fide.)
"The Caput Firmiter of the Fourth Lateran Council (1215) declares: “Those (the rejected) will receive a perpetual punishment with the devil.”
God bless you!