There is no rule about a minute per stop...at least not one that actually holds true. It will differ with each person, camera, film, etc. General rules of thumb are usually expressed in percentages of normal development, and are usually very rough. One exception of which I am aware is C-41 processing of Kodak Portra 800, where the rule is 30 sec. per stop, up to two stops.
36 min. is very excessive if keeping with a standard agitation routine. I can almost guarantee you that you cannot get the highlights of the film to 3200 (3-stop push) with development alone, so all you will be doing is mushing up any sharpness and detail that you do have, which, given a three stop underexposure, is probably not much.
This is why I say it is almost 100% not possible: A true three stop push means that you are taking something that was exposed in a way that would give it a tonality of middle grey with normal development, and pushing it up to a very very light grey (practically white, but with some visible texture) via overdevelopment. This just ain't gonna happen. With most films I have tested, even a "true" two-stop push is barely attainable with development.
Now, this does not mean you won't get printable exposures. They will just be hard to print. It just means that beyond a certain point, pummeling your film any more won't really do much to help you print the negs. It will make the shadows more grainy and washed out, the details less sharp, and the tonal relationships could get somewhat "odd" in the midtones. You are better off developing the film as far as it can go while maintaining reasonable quality, and then intensifying via the Formulary intensifier and/or selenium toning.
As for what I would do...I would probably develop using that Website's +2 guideline (2.25x = 18 min.), which I would say is a good generalization, and see what I got. (Actually, I would develop at 24C to reduce the time.) Then, if I wanted to print any of the pix, I would probably end up trying to get the best print that I could with the original neg., then start trying intensification. In most cases, you can get to + 3 or a little farther more cleanly by developing to + 2 then intensifying.
Just FYI, the film in question is actually Agfa 400 resold as Arista II, if my memory is correct...which it may not be. It could be Forte or Foma if not Agfa. The Arista relabelling specifics can get somewhat confusing, so I am hoping someone chimes in with Arista II's true source.
I know for sure that Efke 400 is Agfa 400, in case you want to get more.
All of this, of course, is assuming that the metering technique was good, meaning that you were actually underexposing by three stops from the "best" exposure. Sometimes you think you need to push but you really don't because certain things are throwing off your best judgment or your meter. Yet another reason to develop to + 2 maximum: you get to see the negs before deciding if they need more density.
Stand development is something which I am sure someone will bring up. A semi stand combined with a push would probably be a great option, but, of course, is all rather iffy without some testing first.
