Wage garnishments have to stop immediately once your case is filed but you should allow time for the creditor to provide your employer with the necessary paperwork to actually cause the stop. If your case is filed the day before your next payday, chances are you're paycheck will still be garnished, as the payroll has already been processed. However, you'll get that money back.
To ensure the garnishment stops as quickly as possible, call the creditor's attorney as soon as possible after filing to let them know that you've filed bankruptcy and that the automatic stay prohibits any further garnishment. Have your case number on hand to provide to the attorney. This puts the ball in their court to take the steps necessary to make sure you're not garnished again going forward. Let your payroll company or HR department know about the automatic stay protecting you from future garnishments.
Keep in mind, however, that they're processing the garnishment based on a state court order telling them to do so. While some employers are able to put an end to the garnishment as soon as they are aware a bankruptcy case has been filed, many wait until they get official word from the court that entered the garnishment order in the first place. This makes sense, as they don't want to run afoul any state court orders, but it means that you'll need to stay on top of them and the creditor's attorney to make sure they get the stop order from the state court quickly enough to stop the garnishment before your next payday.
Don't wait until the day before your next payday to find out whether that's happened; at that point it will be too late to do anything about it. Instead, keep in close contact with someone in your payroll department and - if they're not hearing from the creditor's attorney - follow up with the creditor's attorney again. It's their job to make sure your garnishment stops. Don't be afraid to call and remind them of that if it seems like they're dragging their feet.