Short answers, pulled from the story.
An ongoing series is a comic book that runs indefinitely, unlike limited series which end after a set number of issues. This format differs from one-shots and graphic novels, though collections of graphic novels can sometimes be considered ongoing.
Traditional schedules often dictate monthly or bimonthly releases to maintain reader engagement. Monthly or bimonthly release cycles form the backbone of traditional comic distribution models.
Low sales figures frequently force editors to cancel titles before storylines conclude naturally. Publisher bankruptcy or editorial decisions also terminate series without warning in some cases.
Action Comics has published nearly continuously since 1938, establishing one of the longest-running records. Action Comics accumulated 904 total issues due to weekly releases during 1988, 89.
Detective Comics ran from 1937 to 2011 before resuming publication in 2016. That first volume issued 881 problems between those years, making it the longest continuously published comic book in the United States.