I know of no way to do this using the Messages app and iMessage service. There are probably several ways to send timed or schduled messages via SMS (Which can be received in the Messages app).
The only service I know of off the top of my head is IFTTT.com. You can create a schduled SMS message, or have one triggered by various events. There are probably other services and/or apps that will do similar things. Searching the App Store for terms like ‘delayed SMS’ and ‘auto SMS’ turned up a couple of apps, but the reviews were not encouraging.
If we had more details on what you are trying to accomplish we might be able to suggest other apps or solutions.
TERMS:
iMessage is Apple’s message service, and only works between Apple devices and computers.
SMS is the text message service that works through your celluar provider, and works between almost all cell phones.
Messages is Apple’s app that supports both iMessage and SMS. It will use iMessage by preference (blue text bubbles), and SMS when it can not (green text bubbles). That is, if SMS is enabled in Messages’ preferences.