Unfortunately it is possible. The vocational commitment most occupations require seems to have escaped those who want to teach yoga. The job isn’t taken with much seriousness.
While I acknowledge that it isn’t possible for everyone to change careers, to immerse themselves in yoga etc, my hope is that those who opt to teach as a sideline somehow find a way to bring depth, authenticity, and safety to their students.
And of course sometimes a person starts with what time they have available, then makes a gradual transition to their new career. From what I see around the world, yoga surely needs more dedicated, consistent, well-trained teachers to guide others. How one becomes THAT in spare time, I do not know.
But yes it is entirely possible to hold a retreat in one’s spare time. The begged question is “what potency does a teach muster if they are not fully committed to what they are doing?”.