Do write ups expire?

I received my first verbal warning back in around March for calling off without proper notice. Today I called out again but with plenty of notice, I called out 5 and a half hours before my shift had started. I asked my manager if this would be my 2nd written warning but they didn’t really give a clear answer they just said it would go as an ‘unexcused’ absence. I had heard from coworkers that warnings usually expire after 90 days which i’m not sure if its true or not but if it is there’s no way this would count as a second warning if my first warning was months ago right?