How long does delirium last?

A short post, but rather shocking.

Today I had a talk with the charge nurse at the cottage hospital. I asked her when would a decision be made about whether my elderly Mum’s confused state is delirium (that may resolve) or her new normal. She told me that the ruling used to be that patients would be observed for a month, then it was extended to two months, and now it’s an observation period of 3 months. She also said that if the patient has some other episode, like a UTI or another illness or infection, the clock restarts. She said that she thinks this is just an excuse for not providing any mental health services for inpatients.