I ask this because in 2018 John Isner and Kevin Anderson had a set that latest for 50 games in one set. And in 2010, John Isner and Nicolas Mahut had a set lasting 138 games.
Depending on the competition it is very well possible to have a match last forever. Tennis itself has no rules about a time limit. See the longest match ever played, which took 11 hours in total. Matches without a tie break in the last set are prone to take longest.
I can’t verify the rules for every competition, so here are a few majors:
In Wimbledon they’ve changed the rules to have a tie break after a score of 12-12 in the last set, so it will be a lot harder to have matches lasting that long.
The Australian Open followed Wimbledon by introducing a final set tie break.
That leaves the French Open as the last Grand Slam without a tie break in the final set and they seem to keep it that way.
(The US Open already have a tie break rule)