It's the position of the ATF that you have to have a gunsmithing FFL if you keep a firearm overnight, or if you're "in the business", which is usually taken to mean that you're taking money for doing gun work.
You "can" do gunsmithing without an FFL if the customer stands right there while you do all the work and doesn't leave it overnight.
Even then the ATF may frown on it.
They have no different standard between a full-time gunsmith and a part timer.
That's not the standard, the standard is as above.