If you are a contractor, structure your agreement such that maintenance and support requests outside of the SPECIFICALLY DEFINED scope of work are charged on a per incident cost.
There is a long history (and associated case law) for per-incident support billing. Get behind that.
If you are an employee, get your employment expectations in writing. Do this before you are required to meet those expectations, and ideally in the context of discussing your compensation.
Chasing an employer for unexpected overtime fees sucks for everybody and gives you a bad reputation. Be proactive about compensation.
What is algebra, exactly? Is it one of those three-cornered things? -- J.M. Barrie