As previous posters have stated, AN-related hearing loss can be gradual or quite sudden. My hearing loss was very gradual, probably over 5 years time, maybe more. I started out with excellent bi-lateral hearing and didn't notice the gradual loss in one ear until around 2000. Like many others, I realized that I was 'favoring' my right ear when using the phone. Eventually, I had to admit that I really couldn't hear much out of my left ear and attributed the loss to my age and the fact that, as a radio broadcaster for 25 years, I had tight-fitting earphones next to my ears that were usually quite loud. My wife (a paragon of logic) questioned why, if that was the cause, I was only losing hearing in one ear. I shrugged it off and said I was just happy I could still hear out of one ear! I rejected her suggestion that I see an ENT because, I said, he would just tell me I was deaf in one ear, and I already knew that. She didn't push the issue and frankly, I adjusted quite well to my unilateral hearing situation and no one ever noticed, or if they did, no one ever mentioned it to me. Finally, other symptoms (loss of the sense of taste, loss of equilibrium, intermittent stabbing pain on the side of my head, fatigue) drove me to consult my PCP and he referred me for an MRI - and the rest, as they say, is history.
So, my hearing loss occurred at a glacial pace, others lose their hearing much faster and some, almost instantaneously. There is no 'pattern', although simply holding the phone to your 'bad' ear and seeing whether you can hear the dial tone (this doesn't work on a cell phone because they have no dial tone) is an 'amateur' test that anyone could do. Then, if you suspect you may be losing hearing in one ear, a hearing test with an audiologist is the best way to be sure.