How long can the store hold my iPhone 15 order until I pick it up?

You are asking other end user here, not Apple. All we know is what Apple publishes in this shopping help article:

Shipping & Pickup - Shopping Help - Apple

Under the section "Apple Pickup," It appears you can select a pickup time in SOME stores:

 In some instances, you may also be able to select a window of time to pick up your order at an Apple Store. This Check-In Window will ensure that your order is available shortly after you arrive. Just check in with a Specialist when you get to the store. If you’re unable to come in during this time window, you can pick up your order afterward, but wait times may vary.

So you need to contact Apple, not other end users.

azielMota wrote:

Hello,

If I buy my iPhone15 online and select the US store pickup option, how long can the store hold my order until I pick it up? I'm traveling in 3 days.

Contact the store and ask them.