I'm using the 3x program from this site. I previously purchased a gold membership so that I could move the training around to fit my schedule. It's been pretty good so far. Then again, I've never done a HIM before so maybe I can let you know it's effectiveness after race day!
The beginner HIM program (free) on this site is pretty good.I think it's 20 weeks. I basically followed it last year when training for my first HIM. I did modify it a little due to some early season swim-run races, replaced some of the swims with stuff from master's swim, increased the time of the long rides to allow me to ride the race distance on my long rides, and did a lot more long bricks than the program suggested. Probably not necessary for all, but I felt with my lack of bike background and slow bike speed, I needed that. (Had never done a training ride over two hours before I started it!) You should be well-prepared to start it if you've done the Oly program.
I had to do two attempts at the HIM as the bike course in the first one was cut short due to snow, but I finished strong in 5:53 in my second attempt a month later in better weather. I was definitely prepared!
I have used Matt Fitzgerald plans for 2 previous HIM races and thought they prepared me well. It's nice because he has 10 levels for each distance so you should be able to find one that fits your goals and current fitness. The book that contains the plans is available on Amazon. I think I paid $15 for a used copy a couple of years ago. Probably the best return on my investment since getting into Triathlons!!