Coinbase is a service, your transaction is happening on their internal servers, not the bitcoin network. If you choose to withdraw your bitcoins to an external wallet, that would involve a transaction on the bitcoin network.

So the delay depends entirely on Coinbase, during high volume times I would guess their order fulfilment is slightly slower. Your order will come through though!

I haven't had any transactions be that slow with Coinbase, but I've only used it for a week now. I do know that during busy times, like the other person above answered, it can be slower. If their network is bogged down with requests, it will take longer.

I think it also depends how many confirmations are being used for the transfer. I did a transaction today that was 6 confirmations to completion and it took about 15 minutes, where others were practically instantaneous, and only 2 confirmations.