I started a new town a couple of weeks ago. I'm sure that I've donated at least twenty items and from every category but Blathers still hasn't asked me about the second floor of the museum.
You have to talk to him a certain number of days, so even if you've donated enough items, if you haven't talked to him on enough different days he won't ask about the upgrade.
Also, be sure you don't forget at least one piece of art. My first town's upgraded was a bit delayed because I didn't watch for Redd and forgot that one piece had to be a genuine piece of art.