How long can you safely take Premarin?

My mother-in-law had a hysterectomy 30 years ago. I don't know if she was taking Premarin before that, but she is almost 92 now, and her doctor still has her on Premarin since the procedure. I couldn't take HT because of a breast cancer risk, and was leery of it before that. I opted out of it and am quite healthy. Why would someone be kept on HT for that long? Is it necessary or safe to do? I'm concerned as to what long term effects it has had on her.