The better question is when should you hire a licenced tradesperson.
The quick answer is simple. When required by law or sanity, hire the specialist. You and your loved one’s safety comes first. Generally speaking, if the work requires permits, the inspector will expect that the work is being done or supervised by a journeyman or red seal tradesperson. Just because I am capable of doing something doesn’t always mean it is always in your best interest or mine. If that is the case, I will tell you.
For example, gas lines. I don’t touch them. Hire a qualified gas fitter. Need a new electrical panel, hire a qualified electrical contractor. I could go on, but the point is that there are tradespeople out there who are experts in their field. When it comes to safety, don’t cheap out.