how soon the official offer will be sent out after oral offer?

I had passed the phone interview, on site interview. I received a call from HR saying that they are making an offer for me this Monday late afternoon. He said the offer letter needs to be aproved by CEO. I agreed the salary they offered by oral as well. He said he can make it this Tuesday hopefully. 

So far I have not heard anything from the HR yet. It has been 3 days! Is it normal? May I send the HR an email to ask or just wait?

Waiting online 

Many thanks!