If they bowl 96 overs prior to the scheduled close of play time they continue playing until the scheduled close of play time.
The rule for the County Championship is that play ends after 96 overs have been bowled that day or 18:00 - whichever comes last.
It does happen, not too often over here or in most of the world in Tests, but in games where a lot of spin is bowled, particularly in Tests in the subcontinent it’s quite common.