Read carefully. Once you are certified as a Dive Master, you can enroll in an instructor course and complete it in a couple weeks. A DM in the shop I worked went to a well-known instructor program in Utila and completed it in a month, but he was already an experienced DM when he went.You are right I was being more brash with the time line as they require 6months dive time for instructors but let me find a few that let you compleat every thing in as little as 6 weeks and then you test out after the required wait.
It is not just time. You have to complete requirements at a required level. They don't just hand you a certification after a certain number of days and a certain amount of time.
There is also something to be said about those intensive training agencies. The friend who did the month in Utila described an average of 4 dives a day for that month, with each dive supervised and having specific requirements. You gain a lot more from that than from weekend dive trips with friends. He was a very different diver when he came back from that experience.