Can I get my refund after 15min trial? I have purchased some application before. Later, developers claimed that their application become free, like Link Bubble etc. Can I get my refund? I know that there is a 15 minutes trial after purchase. But I have purchase a Input method application, however 15 minutes trial cannot let me try all functions. And after 15 minutes I cannot get my refund. Can I get my refund back in this condition?

I take Link Bubble as an example, I really like this application. I'd like to purchase this application whenever how much it cost. However there is really a problem that I have purchase one application years ago and never used later. It really bothered me.
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.