I was late to work.

I manage a restaurant I was scheduled to open the store but I overslept and I got to work late. I’m really good at my job so my boss tells me he’s going to give me a write up but he’s too busy so he’ll give it to me the next day. I said ok I deserve that. Next day comes I guess he forgot and still hasn’t given it to me. So my question is there like a time frame that he has to give me this write up for this incident? Or can he basically just give it to me whenever