If you instead have a reputation as someone who is careful about managing their time and can quantify your busyness more concretely, you have a better chance of avoiding the new work. When you say, “I don’t see any really significant swaths of open time to work on something like this for at least three weeks, and in the meantime, I have five other projects competing for my schedule,” it’s hard for someone to rebut you, unless they’re willing to challenge your calculations, or demand that you expand your working hours to accommodate their specific request.
