How common is it that a day trader is a part-time day trader?

It happens more than most think.

Depending on what time zone you are in, you can get in a few hours of trading either before you go to work or after you go to work. Depends on whether you work an AM or PM shift. Most day traders start out doing this to determine if they have the “right stuff” to transition to full time.

From experience, most don’t! Most as in 95% failure rate.