Response times typically vary from 2-8 business hours depending on the severity of your issue. However, the response time is not guaranteed.
No, we do not force upgrades on you. You can continue using PowerBuilder 2017 for example, and migrate at your own pace to a newer version (assuming you don't want to stay in one version until the end of time). Of course, we encourage you to migrate at least when the version is EOL so you can continue to get tech support and bug fixes from us.
In typical circumstances, you can expect one Maintenance Release per quarter in the first 12 months after a major version is released, and then as needed until its EOMM (end of mainstream maintenance).
We make formal announcements in our Product Update webpage, and send notification emails to all the customers that have purchased Appeon products, and all the users that have been assigned with Appeon product licenses. Also, we recommend that you frequently check our product availability matrix at /update.