How much ongoing upkeep is required to maintain a custom-built internal app like this one?

The member said upkeep has not been terrible. The app was built incrementally over about six or seven months (since roughly the end of last year), not as a daily effort. They started with a tax dashboard, then added a bookkeeping dashboard, then a lead-tracking view, expanding the tool as new needs came up rather than building it all at once. The tool also includes built-in checks, such as prompting staff who try to skip a workflow status without completing the checklist.