Send later on ios iphone mail app

Use the Mail app on your iPhone, iPad, or iPod touch - Apple Support

You can swipe down on the email draft to put it at the bottom of the screen while you look at another email message. Just swipe down from the grey line  at the top, and drag it to the bottom. To open the draft, just tap it again.

You can even close the Mail app and when you reopen the draft will still be there ready to send...but there is no set time...

The native Mail app does not allow for scheduling a send in Mail. There are other mail apps available in the App Store, and some can support that task.

Sorry as far as I know you cannot set a time...