Mortgage Bankers should be able to Pre-approve within an hour. My turn time is usually around 1 hr. Just make sure you have all of the required info ready.
Recent pay stubs
2 months of asset statements
A tri-merge credit report will be pulled to check your credit also.
If anyone offers a Pre-approval without asking for this information you might want to second guess their level of service. It is quite important in this market to put the time upfront on all Pre-approvals. This way there are no surprises!