Just one example, I carried out a transfer using Dwolla to go from my bank account to Mt. Gox on Wednesday night, and the Dwolla site projects the transfer to be completed on their end by today (Monday) at 5pm. That's three full business days, not including the fact that there's been a weekend in between. Current status? I still don't see it in my Mt. Gox account.
Dwolla is a cash service. Getting cash into dwolla is what takes a long time. From there, sending the cash already sitting in your Dwolla to either Mt. Gox or wherever is fairly quick. Technically, it happens instantly -- Dwolla's debits your Dwolla account for the amount you sent and then credits the Dwolla account to whom you sent funds.
Now, the method that the exchange uses to then credit your account at the exchange with the funds received will vary per exchange.
Mt. Gox processes Dwolla transactions hourly.
But until those funds are in Dwolla (which you can easily monitor by logging into Dwolla), there is nothing that Mt. Gox can do about it.