In physical age, I'd say Misery has existed for well of a millennium. But coming from life-age and growth I'd say she has the body of a young woman, probably around 19-20 as a slave, but when the curse was lifted she looked around 28ish.
Given that Jenka is Up there in age with Ballos, and still walking around without ever hinting at living in stasis like Misery did, I'd say their magic allows their bodies to age much slower. So I'd put Misery's actual life age at around only 300 or so, if even that, taking into consideration that the crown has only been worn a handful of times, all apparently resulting in failure by the wearer. The Doctor's reign only lasted (game day wise) I'd say a couple of months at the max. And he seemed to be doing extremely well when it came to his plans.
We of course don't know how long the previous wearers lasted, but one could only assume that they didn't last for more than a year each, (otherwise with the power they most likely would have succeeded in whatever malevolent goals they had).
I'd say Misery is 500 at max when it comes to how long her body and mind was able to grow and develop properly. But she is well over a millennium in years of physical existence.