what time postman deliver letter?

I would like to know around what time postman diliver letters in japan?

It depends on many factors, distance from Post Office, size of your area...
When I was living in Nerima, letters were delivered on morning before 11 AM. Now at the place I live in they are delivered at around 3 PM. Too many factors, inaka (countryside) or big cities like Tokyo, Osaka etc...

Ask at your local post office. They will know when the postal worker passes through your area.

Local post offices don't know anything about delivery.
You need to ask 集配局.

I am waiting for some documents from osaka.
I wanted to know around what time postman usually deliver letters.
Seem like it depends on if city is big or small.
City I live in is not very big.

Documents from Osaka usually arrives by the day after it was sent out. If the mail has a reference number, the sender or the receiver can contact the delivery service to have it delivered within a certain time. If it hasn't been sent out yet, the sender can specify the time range of delivery.

Have you asked the sender for details? Are you sure the sender sent it via Japan Post and not by other delivery service such as Yamato or Sagawa?