Goodbyes are an interesting thing on Doctor Who: They're built into the package deal, but since they don't exactly advertise the expiration date upon casting, they still have the ability to come as a surprise. Hence the mild "oh!" I emitted after finding out that Jenna-Louise Coleman is likely leaving Doctor Who. Followed by an "ah, yeah."
The amount of time a companion spends accompanying the Doctor around, well, space and time fluctuates on a case-to-case basis. Talking exclusively about the modern reboot of Who, you've got four main companion examples: Billie Piper's Rose, who stayed on for two seasons and two Doctors; Freema Agyeman's Martha, who stayed for one season and one Doctor; Catherine Tate's Donna, who stayed for one season and one Doctor; and Karen Gillan's Amy (often accompanied by Arthur Darvill's Rory), who stayed on for two and a half seasons, vacating the companion spot(s) in late 2012.
Coleman, who plays companion Clara on the show, has occupied the mantle ever since, including during the departure of Matt Smith's Eleventh Doctor. Reports indicate that if she does end up leaving the show (it's heavily, heavily rumored at this point, but not explicitly confirmed), it will be during this season's Christmas special. That'll mean that Coleman will have spent a season total with the Doctor — though the actual timeframe in-world is a bit more...wibbly-wobbly. The Ponds spent over a decade traveling with the Doctor, after all, but it only translated to 2.5 seasons.
Despite the fact that it's built into the routine, losing a companion's often an emotional experience for both audience and character. Let's just hope that Clara leaves of her own accord — he does seem to have a life going for her on Earth — and that she doesn't end up in a graveyard somewhere or floating on the wind in leaf-form.
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