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DM's get free air including Nitrox.
tenover, the work requirements are going to vary with the type of operation through which you are doing your DM, and perhaps a bit what your goals are.
For my DM, I assisted with classes -- that meant I showed up at the academic sessions twice a week, helped issue equipment to the students and solve problems. I sat through the lecture sessions, corrected quizzes and knowledge reviews, and helped students get their gear to their cars. At the pool, I helped carry gear and supervised gear assembly, and I then dove with the class. Afterwards, I'd go back to the shop and help check in the class equipment, rinse it and hang it up. These days began about 6pm and ended at midnight, and that was Monday and Wednesday of each of three weeks.
On the weekends when we did OW dives, I went to the dive site, helped put in the float and line, and helped move gear and supplies for the class and get them set up near the beach. I helped students with their equipment. I was sometimes asked to do the site briefing. I dove with the classes, then helped students swap out tanks and supervised to make sure they stayed warm and were okay. We did the second dive, and then I helped everybody get their stuff back to their cars, and brought back in the float and the extra weights and all the supplies we'd unloaded. These days began about 7am and ended around 2 to 3 in the afternoon. This was one day of each of two successive weekends.
Other than that, I was not expected to work for the shop, but I did my DM through a local dive shop here in Seattle, not a residential program like some of the DM classes in Asia are.
To the OP -- I have a vague memory that I was unable to assist with classes as a DMC until I had completed some of the academics -- I'll have to go review the standards, but that's what I remember.