What type of classroom, confined, or open water work is required and about how many days?
I can tell you what is required, I can tell you how many days were involved for a DM class I just conducted, in 2019. What I cannot help you with is how long it will take with the LDS that you are going to work with.
The program involves academics, skill development / proficiency, and experiential learning. The requirements include:
Knowledge Development
9 Topics covered in online independent study and classroom sessions
Demonstrate successful performance on a two-part Final Exam
Develop and submit an Emergency Assistance Plan
Waterskills Development
Watermanship and Stamina - timed and scored. You must get a total of 15 points across 5 activities
400 yard swim
800 yard snorkel swim
15 minute tread
100 yard inert diver tow
Problem-solving exercise
Diver Rescue Skills (essentially, mastery performance of Rescue Exercise #7)
Dive Skill Demonstrations - this is where they develop the ability to perform, at a 'Demonstration Quality' level, 24 skills (the skills that are taught in the PADI Open Water Diver course, such as Regulator Recovery, Mask Clearing, No Mask breathing and swimming, etc., etc.).
Practical Applications
Practical Skills
Skill 1: Dive Site Set Up and Management
Skill 2: Mapping Project
Skill 3: Dive Briefing
Skill 4: Search & Recovery Scenario
Skill 5: Deep Dive Scenario
Divemaster Conducted Program Workshops
Workshop 1: Scuba Review in Confined Water
Workshop 2: Skin Diver Course and Snorkeling Supervision
Workshop 3: Discover Scuba Diving Program in Confined Water
Workshop 4: Discover Scuba Diving Program – Additional Open Water Dive
Workshop 5: Discover Local Diving in Open Water
Practical Assessments (aka Internships)
Open Water Diver students in Confined Water
Open Water Diver students in Open Water
Continuing Education Diver students in Open Water
Certified Divers in Open Water
I have my DM candidates complete Divemaster Online, and Dive Theory Online, instead of me conducting the classroom segments. It usually takes them about 8-10 hours for each course (working at their own pace, and times). Some are quicker, some are slower.
We completed the Confined Water work (Waterskills development, skill scoring, and Workshops 1 and 3) in seven 3-hour pool sessions. Now, in every DM course, I expect them to get in the pool on their own to practice, outside of the scheduled times, so there was, in reality, more pool time involved, but I can't tell you exactly how much more. As it turned out, all of last year's DM candidates had Deep Diver certification already so I didn't have to do a Deep Dive Scenario, and 3 of the 6 had Search and recovery Diver certification, so I completed the S&R Scenario with 3 of them in a ~3 hour session built around their AOW internship.
We completed the Open Water activities (Workshops 2,4, and 5, and Rescue Exercise #7 in OW) in two 6-hour days. The Mapping Project is conducted on their own, and usually involves each DMC completing 3-4 OW dives (hour long) of the area, working in buddy teams. They complete the requirements for Skill 1 and Skill 3 in their internships. And the Internships take as long as the classes they are working with require. DM candidates are expected to participate in full CW (variable, usually 4 half days in the pool), full OW (2 weekend half days)and full AOW classes (2 weekend days).
Again, this is how I did it / do it. Other Instructors may take more time, while some may take less.
I won't offer comment on whether you should or shouldn't pursue it, other than to say that I pursued Divemaster training because I wanted to improve my diving skills, and - at the time I took the course - I had no intention of becoming an Instructor. I accomplished my goal.