For those commenting on "black lung" (like
@goodhunter and
@Ultenth) this is more likely a metallic ore mine than a coal mine, and as such, that wouldn't be an issue (and even if it were coal, there won't be an issue with that, because it is a slice of life manga, but we can chalk it up to a magical protection or otherworldly phenomenon that prevents it, for an in-universe explanation). He is just saying that he feels restricted, because he has a set work schedule (even if it seems designed to result in more production and fewer hours, overall). He doesn't seem to be the only employee, but does seem to be the only one trying to skip out.
@icekatz This doesn't seem to be about there not being any breaks, as she mentions that he is taking up some of his lunch break by doing this. With the 8-5 schedule, and a mention of lunch, it appears to be an 8 hour work day with 1 hour of break time during it. Perhaps that is 1 hour for lunch, and perhaps it is 30 minutes for lunch, and 2 15 minute breaks. The fact that she has limited it to 9 hours of being at work and only 5 days a week also shows allowing for more time for recuperation outside of work, so she clearly isn't trying to overwork them at all.
But she objected to him trying to skip out on work, and that he was drinking while doing so. Somebody working in a mine becoming inebriated is a safety hazard for themselves and others around (especially since they are likely using pickaxes or some other sharp equipment). As such, her concerns seem completely valid, and taking what the guy says at face value, when he also said that he only wants to work "when he feels like it" doesn't make much sense, especially when seeing how competent she is in so many areas. We even get told that he had been "spoiled" and then resorted to "complain[ing], skip[ping], and mak[ing] excuses."
Really, though, the fact that he is consuming alcohol in the middle of a manual labor job is a clear sign that this goes beyond "taking a breather" to make himself more efficient again. (In fact, you can easily find multiple articles with a basic Google search that shows that consuming alcohol during work lowers productivity to counter your "productivity" argument). The most plausible conclusion is that he was trying to take extended breaks, regularly, rather than just occasional breaks to avoid overwork.