If we take her story at face value and she is a previous incarnation of the Doctor, then she's got to be pre-Pertwee, as I believe the Third Doctor was the first to wield a sonic screwdriver. And since I believe we saw the First Doctor regenerate into the Second, she's either pre-Hartnell or perhaps somewhere between the Second Doctor and the Third. (I don't recall there being an actual regeneration scene between those two - but I could be wrong.)
I'm personally of the opinion that the "Ruth" Doctor somehow isn't in fact the real Doctor, as it would seem odd to cast the first female Doctor and then write up a plot-line where nope, she wasn't the first female Doctor after all. (I'm applying "meta-game" logic here, I admit.) I do recall in "The Doctor's Daughter" the Tenth Doctor ended up with a female clone patterned after himself; it could be something like that or maybe the "Ruth" Doctor is from a parallel dimension. That is, after all, where the current batch of Cybermen come from (another Tenth Doctor group of episodes - funny how much of this season is calling back to Tennant's reign).
Johnathan