Not sure about MTGOX, but Dwolla usually takes 3 business days. Sometimes less.
Is Dwolla to Dwolla instant ? I don't see why it shouldn't be. If any transfer lasts for days I don't see how it could ever challenge PayPal. But I know they're already using it in some retail stores, so it must be instant. If MtGox doesn't do a Dwolla to Dwolla transfer but a Dwolla to bank transfer that's their problem...