When to Fill Out a USPS Address Change Form
Since it can take up to 10 days from the time you turn in your change of address form until mail starts getting routed to your new location, try to submit your request two weeks before you move. If you want to start forwarding your mail ahead of time, you can do so up to three months before you actually move. Don't do this unless you're absolutely sure there'll be someone there you trust to collect your mail and set it aside for you.
Permanent or Temporary
If you know that your move isn't permanent, you can specify on the form that your address change request is only temporary. You'll be asked to provide the date that you want the forwarding to stop and regular delivery to your permanent address to resume. This often comes in handy if you take up seasonal residence at a vacation home and don't want to return from vacation to a high pile of past-due bills. Otherwise, your mail-forwarding request will be kept on file at your old Post Office for 18 months. By then, you should have had plenty of time to update your new address with everyone.