They'd like it to be. I have a HS classmate who played M. Firmin in the Phantom of the Opera on Broadway for thirteen years running. That's eight shows a week, two on Saturday.
And before Broadway, he did the same part, Firmin, for another thirteen years with a traveling roadshow of the Phantom. I used to kid him that, after 26 years doing the same part, he might finally get all the lines memorized.
And before Phantom, he was on Where the Heart Is a CBS TV soap opera of the 1960s. And he directed the Fantastiks in San Francisco.
And coups de gras, if you've ever seen "Escape from Alcatraz" with Clint Eastwood, my HS classmate is the one who sprays Clint with water in an isolation cell. He has all the fun.
But my point, for him, acting was certainly a full time thing. So the answer is yes, acting can be a full-time job.
But I think even he will agree that he was fortunate. Most actors don't have full-time projects like he did. And most fail to make a legitimate living at acting; so they have other jobs.