I have looked at how long Clara has been in the programme and now I am looking at how close she is to becoming the longest-serving companion, and what milestones fall in Seasons 35 and 36.
I have previously defined how I determine the notional number of episodes, to take into account episodes being different lengths in different eras, and the existence of the specials.
Also, I assume there will be no returns from former companions which would increase their number of notional episodes. So, as at the end of Last Christmas, Clara was on 56 notional episodes, putting her in tenth place.
Season 35 opens with a two-parter, The Magician's Apprentice/The Witch's Familiar. By the end of the first episode, Clara has overtaken Rory Williams and is the ninth longest-serving companion.
The third and fourth episodes form another two-parter. As the closing credits appear on the mid-story cliffhanger, Clara ties with Rose Tyler for the eighth longest-serving companion (and hence the second longest-serving 21st century companion).
The fifth episode is The Girl Who Died, followed by The Woman Who Lived - these episodes may be a two-parter.
The seventh and eighth episodes form a two-parter involving UNIT and the Zygons. By the end of this, Clara has overtaken both Amy Pond and Tegan Jovanka, to become the longest-serving 21st century companion, and the sixth longest-serving companion of all time.
The eleventh and twelfth episodes form a two-parter. Clara enters the season's final episode having pushed Barbara Wright, Ian Chesterton and Jo Grant into joint-fourth place.
Clara begins Season 36 on 83 notional episodes. The end of the fourth episode of Season 36 - if she is still there - would see her tie with Sarah Jane to become the joint-second longest-serving companion and the joint longest-serving female companion.
Assuming that Season 36 has the same pattern of 12 episodes, and followed a 2016 Christmas special, then Clara has to wait until the fifth episode of Season 37 (autumn 2017?) to overtake Jamie McCrimmon and become the longest-serving companion.