According to the purchase I did, I was able to use SongKong Pro forever(!)
See: 
Now - after updating the app - it greets me with a different story:
Confused?
I’ve been updating the app since a long time past that update-date mentioned in the about, until this latest update 9.4?
You can use Songkong version that you purchased forever, but free version updates only available for limited time.
After that time at any time a small version payment let’s you update to latest version if version period expired.
If you found you could update without making payments that was because for some licenses there was a bug that prevented expiry being enforced that is now resolved.