Author here. I don't work full time but I'm a master's student. I had a decent enough chunk of free time to dedicate to this without too many serious obligations. It's hard for me to estimate time per day, as there were some week-long periods where I did very minimal study when particularly busy, and other periods where I'd spend 20+ hours a week of solid effort on it. I think achieving this is probably outside the scope of someone who isn't extremely abnormally self-motivated and efficient and who has a family, long working hours, etc. That being said, I think most people can find the time to reach a moderate conversational level in a year without too much trouble, assuming they work efficiently.