By the time you do something....

Hello,

Could you please check the underlined sentence? Is it correct? If it was a dialogue the woman would say: By the time you receive my letter I will be deador I will have passed away.

The distinctive feature of the disease lies in the fact that it has so-called rejuvenating effects. A woman starts to look younger before people’s very eyes, but after such an effect one has very little time left just a year and a half or even one year which is the maximum time. Now her time was coming to an end. She was in hospice at the moment and by the time he received her letter she will/would already have been gone.

Thanks!