My best advice is keep it simple. From the user's perspective, subscription generally means web hosted service of some sort in today's software environment. I

My best advice is keep it simple. From the user's perspective, subscription generally means web hosted service of some sort in today's software environment. If this doesn't work for you, I'd look at an annual license. Bug fixes are made available all year but a new release with new features (with or without source code) is done every January. Older versions are no longer supported. Essentially, each new release comes with a new support contract. This should be easy enough for most users to wrap their head around.