While we've (Array) certainly retired themes in the past, I wouldn't say we've ever retired them on a "regular basis." This would imply that it's part of our collection's cycle, but in reality, we've only retired themes circumstantially in the past. As we've transitioned and grown, it made sense to focus on the themes that gained the most traction and were favored by our customer base.

Every theme company operates differently, but I would venture to say that everyone wants to make sure they're operating as optimally as possible. For some, this means rolling out new features, others may refocus their efforts on improving existing themes and some may retire themes. There isn't one particular approach that fits every theme provider.

At any rate, it's important to make sure you support retired themes until they've fulfilled their obligations to a user. If you've sold a year of support with the theme, you should honor that commitment if you want to retain and build trust from your customers.