When you've made a bad hire
Most owners wait three months too long. The math says move sooner.
You usually know by week three
The pattern shop owners describe is identical: you knew in the first three weeks, and then you spent three more months hoping you were wrong. You weren't.
Have the conversation first
Direct, specific, in writing: here is what isn't working, here is what 'fixed' looks like, here is the date. Most bad hires don't fix it. A few do. Either way you know.
When to let them go
When the gap is attitude, effort, or honesty, move fast. When it's skill that can be coached, give a defined window and resources. When you're firing the same person for the third time in your head, you've already decided.
Take your version of this question further
This is one operator-tested angle on the question. Your shop, your size, your trade, and your team change the answer. Ask your specific version inside Ask a Shop Owner to get a response grounded in how owners like you actually handled it.