To my understanding cohabitation alone isn't enough for a common law marriage. As long as you don't refer to her as your wife and your place is under your name, then you should be safe cohabiting as much as you want. If she's contributing to bills of any kind then you should claim that as "rent", so that way if she ever tries to claim your property then you can say she's just a tenant and evict her.

buy her a shit-box of her own as a birthday present

Even if it is a shit-box, that's more generous than any gift I would give.

To my understanding cohabitation alone isn't enough for a common law marriage. As long as you don't refer to her as your wife and your place is under your name, then you should be safe cohabiting as much as you want. If she's contributing to bills of any kind then you should claim that as "rent", so that way if she ever tries to claim your property then you can say she's just a tenant and evict her.