I believe I’ve been getting warnings since my first Phonaks when I was a teenager about 30 years ago. Always 1-2 hours of warning with a beep every 30 minutes. I think they may get less accurate with time - my last HAs which were almost 10 years old by the time I replaced them were giving me warnings 1-2 days before actually dying at the end 
Edit: I’ve been wearing BTE UPs my whole life and I wonder if that might be a factor in how the warnings work.