So, time to update my list of the longest-serving companions.
First thing to note is that it depends what you mean by Clara Oswald. Do we count Asylum of the Daleks's Oswin Oswald, or The Snowmen's Clara Oswin Oswald? One of my rules is that I count episodes where a past or future companion appears, as long as it is not a cameo. Although Jenna Coleman's name did not appear in the opening credits of Asylum of the Daleks (to avoid spoiling the surprise), Oswin is a form of Clara from when she entered the Doctor's timestream in The Name of the Doctor.
If you saw my last post, you will see that I have a rule for counting the number of episodes:
By number of episodes, I count a traditional 20th century episode as a single episode. The 21st century one (except for the specials, Last of the Time Lords, Journey's End and The Eleventh Hour) and the ones from Attack of the Cybermen to Revelation of the Daleks I count as 2 episodes each. Due to their length I count the specials, Last of the Time Lords, Journey's End and The Eleventh Hour as 3 episodes each. The Five Doctors counts as 4 episodes.
So, following this, I count The Snowmen, The Day of the Doctor, The Time of the Doctor, Deep Breath, Death in Heaven and Last Christmas as equivalent to 3 episodes each, and the others as equivalent to 2 episodes each.
Season 33 saw Clara in 10 adventures - Asylum of the Daleks and then from The Snowmen through to The Name of the Doctor. That is equivalent to 21 episodes. When we add in the final Matt Smith adventures - The Day of the Doctor and The Time of the Doctor - we are at 27.
Season 34 is equivalent to 26 episodes (remember that Deep Breath and Death in Heaven are each equivalent to 3 episodes), so this put Clara at 53. Last Christmas brings her to 56.
This puts her in tenth place in our list of longest-serving companions, having overtaken Susan Foreman while at the North Pole.
We know Season 35 has 13 episodes, which I will assume are the normal 45 minutes one. This counts as 26 episodes in the system I am using. So, by the end of the next season, Clara will be at 82 episodes - in third place behind Jamie McCrimmon and Sarah Jane Smith.