When should I hire?
The signal isn't 'I'm busy.' It's that revenue is being left on the floor and you can see it.
The wrong reason to hire
Being tired is not a hiring signal. Tired owners hire the wrong person, train them poorly, and end up resenting the hire. Being tired means you need a vacation, a different process, or both, not necessarily another body.
The right signals
You're turning work away you would have taken a year ago.
You're missing follow-ups and revenue is visibly slipping.
You're personally doing tasks at $20/hr that pull you from $200/hr work.
Run the math first
A fully loaded employee at $20/hr costs you closer to $30-35/hr after taxes, insurance, and unproductive time. They have to generate at least 2-3x that in margin to be worth it. If the math is tight, fix the process before adding a person.
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.