I think the 'still cooking' line and a few other points in Who's history allow for the possibility that we may not even get a single Billie episode as the Doctor.
In the 11th Hour he said he was still cooking, and in The Time of the Doctor we get 11 resetting to his young self 'this is just the reset'.
In The Christmas Invasion he loses a hand and regrows it because he's still within the first 15 hours of his regeneration cycle.
Throw in Ten channeling his regeneration energy into his severed arm, River using her remaining regenerations to save the Doctor, River also suggesting she could control her regeneration ("I'm focusing on a dress size" and dialling down her age to freak people out) and of course going all the way back to 1979 we have Romana trying on new bodies like they were outfits* and you can see where this could go.
The Doctor staggers back into the Tardis, she finds a mirror, looks at her reflection, see's it's Rose and goes "No! No, no, no, no...NO!" scrunches her eyes really tight and regenerates again into Dan Stevens or Olivia Colman, or Riz Ahmed.
Might be a bit disappointing so assuming Billie is the Doctor it'd be fun if she got at least one adventure within the first 15 hours before deciding that looking like Rose would just be way too distracting.
*And of course Destiny of the Daleks also establishes a timelord consciously or unconsciously choosing to look like someone they've met before, long before Five or Eleven regenerated into familiar faces.
