how long does it take to get burn in?

My Note8 was set to auto, at about 25%. It got burn in before the Note9 was launched. I think even before the S9 released.
I did the same on my Note 8. Had it for two years, no burn in. I gave it to my daughter when I got my Note 10+.
 
I did the same on my Note 8. Had it for two years, no burn in. I gave it to my daughter when I got my Note 10+.

I had mine for 2 years too. The burn in wasnt crazy like you could find on demo units but it was very noticeable in a bright app.