Ask HN: On premise applications, what do you include in your support contracts?

I'm trying to determine what common practices for support are with applications that are hosted in a client's data center.

Without getting too far into specifics, let's say that you sell a web application based on the Microsoft stack, so you've got Windows Server 2012, .NET 4, IIS 8 and SQL Server 2012 (this should apply equally to other stacks). Your clients provide you with the necessary VMs and you install and configure your application to meet their business requirements. You charge an initial fee for the license and an annual support fee to deal with updates and support requests.

For some period of time, your application operates smoothly - but Windows updates get applied or an update for SQL Server comes out, which may require updates to your application, but may just be prudent to install due to performance or stability improvements. Now you've entered a grey area where you could be charging an additional system management fee or you could push this work back to your clients.

Can anyone describe their approach to this situation? What has worked for you or what has backfired?

Many thanks!