How long does standard flat rate shipping take from California to Oklahoma?

The standard time is 3 to 5 business days from the time the mail gets to the post office.

This is a pretty vague question. There's no way to decide how long the item will be in transit without mentioning specific cities from where the item is being sent from and to. Also, "standard flat rate shipping" is a generic term. Unless a specific courier (such as USPS, UPS or FedEx) and shipping service (such as "Ground" "Express Mail" "2-Day Air") are mentioned, there's no way to tell.