I got my DM in 1990. It took me about a year. I started with someting like 100 dives and completed with around 180.
In my estimation, the prerequisites are not nearly difficult enough. I say this given what I have seen form too many "90-day wonder" DM's, AI's and instructors. Also, knowing that there are shops, resorts, instructors out there that are going to provide nothing but the absolute minimum, there should be more rigid standards inplace.
The RSTC, PADI and most major training agencies are not seeing or are ignoring the problems. I think that long term, this trend of making every certification level easiser/quicker to attain, will bite the industry in the butt.
I think that DM candidates should be required to have 150 dives minimum to begin, 250 to complete, at least 20 deep, 15 night, .....I could go on. Additionally I think that a skills assessment should be done; not just swimming and treading water, but buoyancy control, finning techniques, control of panic situations, etc.
Imagine a newly minted DM, (60-70 dives in career) assisting an instructor and one other DM with 12 students on OW dive 1. Platform is at 28', visibility is <5', water temp 75 degrees. Three divers exhibited nervousness during pool training requiring additional pool sessions, but were able to meet agency "standards adn mastery".
This DM is watching one end of the arc of students while the instructor and other DM are assessing skills on the other end. Now the 95 lb lady one the end starts choking for some reason, and this new DM swims to her to assist. She spits out her reg, pulls off her mask and gets a death grip on her power inflator. Then, just to make things even more fun, she pulls the new DM's reg out of his mouth, yanks off his mask and starts hitting him, all the while trying to set a new breaching record.
I've had this happen to me when DM'ing. Luckily I had over 300 dives and it was not the first case of panic I'd dealt with. I grabbed her BC, locked my legs around the platform rail and started searching for her BC's emergency dump. Fortunately for both of us, the other DM came over and got control of her. I was able to roll backwards, get my reg back in, grab my spare mask, put it on & clear, find my other mask. The lady gained composure and was able to complete the dive later.
Would I have reacted the same way with the agency required minimum "experience" as a new DM? Doubtful. True, some people would be exceptional and handled it well. Most however, I believe, would not. Due to the responsibility entrusted and placed on diving professionals, the standards need to be higher.