Can I get my refund after 15min trial?
You can get a refund from the app's page of the Play Store within 2 hours from the purchase. If 2 hours have passed, the official guideline by Google is:

Contact the developer
If Google Play's 2-hour return period has passed, the developer is responsible for deciding if you are eligible for a refund. Contact the app developer directly and ask for a refund. You can find an app developer's contact information listed on an app or game's detail page on Google Play.

and:

Optional: Contact Google
If the developer hasn't helped, Google may be able to. Contact our support team for help solving your problem.