How long can I use Final Cut Pro? When should I pay for this software again?

You can send me PayPal any time you like.

No, seriously. Since 2011 FCP has been a one time only purchase. No subscriptions. No update fees. Never. So far. Apple might change that at any time, but why? They really don’t need the tiny amount of revenue it would generate in relation to all their earnings.

This is incorrect information. As I said, you have never had to pay again once you've purchased the application. It doesn't matter how many computers, or many versions of the OS, or how many versions of FCP there have been (there have been 43), you've have never had to pay anything beyond the initial purchase price.

When you upgrade you new Mac OS! That usually when they introduce a newer version for new Mac OS! However if enough bug complaints in Feedback - Final Cut Pro - Apple! Then when they might fix the bug if they get enough complaints! So as long as you don’t upgrade in future you’ll never have to pay again!