I avoid selling hours. Instead a maintenance contract should sell a solution, e.g.
- they have an actual contact address
- you send them new versions of your software (nothing in GPL forces you to publish new versions),
- you send them database updates necessary to reflect legal changes of the software
- you help with database migrations, when new software version require them
- you might also sell hosting, including the above service, backups, security updates, ...
Create a nice pile of things that have a good value for the customer, and either are easy to automate (like hosting), or likely don't eat to many hours. You'll lose the ability to invoice your customer for the 10 minutes mail or phone help, but you would need more then 10 minutes for book keeping and invoicing of those small things. So better sum them up, if you want to keep contact to a good customer. As funny as it sounds. The annual maintenance invoice was often the starting point for a custom asking me to solve a total different problem.
Create a nice pile of things that have a good value for the customer, and either are easy to automate (like hosting), or likely don't eat to many hours. You'll lose the ability to invoice your customer for the 10 minutes mail or phone help, but you would need more then 10 minutes for book keeping and invoicing of those small things. So better sum them up, if you want to keep contact to a good customer. As funny as it sounds. The annual maintenance invoice was often the starting point for a custom asking me to solve a total different problem.