I guess the hard part of the answer is that if you are sending ether over an existing account then it would take as long as you took to sign up for Coinbase or go through a wire transfer.
The harder question, that I do not yet have a good answer too, is for transactions where you send funds from one ethereum address to another. If the sender’s eth wallet has been compromised and/or they have chosen to use a coinbase-like service – how much time does this take? In practice who would send funds from their Coinbase account to an ether address? I imagine someone trying to buy bitcoin with credit card or something, still very much in its infancy so maybe coinbase will be offering bitcoin obviously but also may well start adding other coins at some point really.