Without a lot of cutting and pasting - replying to several different posts (sorry if I jump about a bit from topic to topic):
My experience is training at a dojo and organization where people do fail tests, and that does indeed to some extent take care of the question of how often is enough. On the one hand people will train harder when they know that failing is an option, and on the other hand if someone is ready to test despite only training twice a week then that will already be observable before testing. That works for us I think because were a small organization where everybody knows everybody and the standards are pretty clear.
Actually setting a clear testing standard and sticking to it would probably go a long way towards achieving the items on Ledyard sensei's list. Good luck to anyone trying to formulate clear testing criteria though.
Someone said that the fact that people have lives outside of aikido shouldn't bar them from testing for shodan... why not? If aikido isn't a priority, then how much does another grade really mean to that person? Of course it's nice, but isn't it more important in that situation to get recognition for the effort and time that has been put into work, or family, and so on, from the people at home or at work? Or should people get aikido grades for being such good parents and hard working employees? I think there are more appropriate rewards that one could think of...
About teacher training: I trained to become an Alexander Technique teacher at a three year course with just ten students at that time. We all paid for the training ourselves, the school doesn't get any kind of subsidies. I wonder if it would be possible to do something like that in aikido, to have some sort of intensive training course for people who want to become instructors, and whether or not that would do anything to solve some of the problems Ledyard sensei writes about. (I know some dojo have some sort of uchi deshi programs, but I don't know how long people usually take part in those, and how much the intention really is to become a teacher?)The problem of course is that having done such a course people would probably want to make some kind of a living teaching aikido, and that's almost impossible.
I've talked with my teacher about starting my own dojo. The reason I don't want to is because the difference to me is so clear - my teacher teaches aikido for a living, trains every day, I do aikido next to two other rather demanding disciplines, I don't train every day. I think people are much better off training with my teacher.
To return to the original question - for a brand new beginner I think two times a week is enough. Until you have some little idea of what aikido is and how deeply you want to throw yourself into practicing it. After that, you have to decide what you want to achieve, and train accordingly.