Who was the longest standing Sith Master?

Question

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.

Answer 1

Legends

Outside of Bane's Order (which was the only Sith Order that followed the Rule of Two), the longest reigning Sith Master was the Sith Emperor Tenebrae (also known as Vitiate and Valkorion), who reigned over the Sith Empire for over 1300 years (4980 BBY to 3630 BBY).

Within Bane's Order, the known length of time for each Sith Master's reign is as follows:

• Darth Bane: 20 years. Bane was the Sith Master from 1000 BBY (starting with the Seventh Battle of Ruusan) until his death by Darth Zannah in 980 BBY.
• Darth Tenebrous: 100 years. Tenebrous killed his Master in 167 BBY and was killed by Darth Plagueis in 67 BBY.
• Darth Plagueis: 35 years. His reign began in 67 BBY and ended with his death at the hands of Darth Sidious in 32 BBY.
• Darth Sidious: 36 years (starting from the death of Plagueis in 32 BBY), longer if you include the time of Sidious' mentorship of Maul (we don't know what year that began). Sidious' reign ended in 4 ABY at the Battle of Endor.

Darth Tenebrous had the longest known reign of the Sith Masters in Bane's Order. However, we do not know how long many of the other Sith Masters in Bane's Order reigned, so it's possible Tenebrous was not the longest reigning.

Canon

There is insufficient information to answer using strictly canon information. We don't know how long each Sith Master reigned (not even Sidious, because we don't know from canon when he killed Plagueis).