I'm not asking who was the oldest in age, or who survived the most years being a practitioner of the Dark Side, but rather which Sith Master was given the embarrassing burden of living the longest while having an apprentice or multiple apprentices. Sidious had Vader under his wing for 20+ years, and this seems like a long time given that the end result must always be that the self-centered apprentice rise up and kill their Master--gaining all the power and status--much to their Master's dark desires.
So which Sith Master had the longest-lasting mentorship? Legends answers are acceptable, as I don't think there's enough new canon to really pull from.
There's this answer in case anyone needs a refresher on the recorded Master/apprentice pairings. For the cases where there was a tie, multiple apprentices (trained in secret or outside of/before Bane's Order) or the Master actually won (the list in the link shows this rarely, if ever, happens), the years spent with all apprentices should be added up with the first one for the grand total.