I think I got used to it in maybe a week or so. Far enough to one side that it was not where I needed to see anything as I rode straight ahead or slight curves to one side.

And since I have two eyes, it took a while for my brain to figure out which eye I wanted to use, the one that saw what was in the mirror reflection or the other eye that saw what was behind the mirror.

I commuted for decades on motorcycles, they had good mirrors on the handlebars, I did not have to move my head very much to see in the mirrors. That made me want to have at least one mirror on a bicycle too. But I did not want to have to move my head much to see in the mirror. Thus, with drop bars, a handlebar mirror was not what I was looking for.

The only thing I do not like about a helmet mirror is riding away from a rising or setting sun, with the sun in the mirror, thus shining in my eye. Sometimes i reach over and change the mirror angle so that I can't see behind me any more until the sun is no longer a problem, then I re-aim the mirror again.