It is a good question for those who are considering going that route. I believe it depends upon three things: where you live, how much money you have, and what agency you select.
Location: I live in Colorado, where we don't have much at all in the way of technical diving locally. For most of my training, I had to travel with like-minded people 6-7 hours to New Mexico to dive in a deep sink hole. That meant spending three nights in a motel every time I wanted to dive. I had to find a time when there was enough like-minded people to do the trip. Alternatively, I could travel a few thousand miles to some other place. That made it very hard to do the needed dives. It is still that way. It is very hard to find other technical divers here to make such a trip.
Money: The equipment, training, gases, and dive logistics (including travel, boat fees, etc.) can all be very expensive. I know people whose pace of training is based solely on the number of thousands of dollars they can devote to this annually.
Agency: While most agencies have similar requirements, others have vastly different requirements for the number of dives you have to do and the overall experience you have. Most agencies have this philosophy: We will train you until you reach the point of being a safe and competent diver with all the skills needed to complete the dives for which you were trained. If you choose to go on from there and achieve expert status, it is up to you. Other agencies have this philosophy: We will train you and keep training you (while you continue to pay for that training) until you reach expert status for that level of diving, at which point we will certify you to do it on your own. I did the math with the agency I was training with for years and made the decision to switch to a mainstream agency. I completed my advanced trimix training with the new agency and did quite a few advanced trimix dives. I also got my cave certification with another mainstream agency. I am now an instructor with the agency to which I switched for trimix training. If I had stayed with the first agency, I wold probably still be working on the normoxic trimix certification, and I would probably need several more years to get to full trimix. (There was no exaggeration in that.)