Pressure Cooker to Stovetop conversion? I want to make ramen, in particular, tonkatsu ramen. My cookbook is in Japanese and even though I'm reading everything else fine, a part of it is confusing me (but no worries, I'm not confused about the Japanese part).

I'm supposed to make the broth (I think the book is saying "stock" here, but I'm going with broth because the way it's written makes this a little unclear) at a "rapid boil," which I put in quotes because this cookbook follows this up with some ridiculous claims. I'm going to do it on my stovetop. The cookbook says to do rapid boil the broth on the stovetop for 90 hours. To put it frankly, that is ridiculous. A rapid boil for 90 hours is... well, I'm pretty sure ramen can be made in a day and I'm nearly 100% certain I shouldn't be rapid boiling broth (it should probably be simmered if it's stock). The book also says you can make the broth in a pressure cooker in an hour and a half. That's more reasonable, but I don't have a pressure cooker.

There are a couple mistakes in this book, whether because I'm not translating correctly or because there are actual mistakes I'm not sure-- rapid boil, for example, is used a lot in places where I know I shouldn't be doing that, which makes me think my translation is wrong-- but it absolutely, positively says 90 hours for stovetop.

I highly doubt that I need to make this ramen broth by boiling for a whole 90 hours on the stovetop. I'm certain it could be done in less time. Is there a conversion process between pressure cookers and stovetop? What I mean is, is there some way for me to determine how much time it would take to cook something on my stove top based on the directions I was given for the pressure cooker? Thus far I've only gotten "keep temperature on low and keep it at a slow boil," but I'm wondering if there's something more concrete than that.
There is no simple conversion, as you are looking for outcomes.

Typically what can be done in a pressure cooker for stock or braise type applications in 15-30 minutes might take 3 hours on stove top.  An hour and a half in a pressure cokoer might be analogous to a full day (call it eight to ten hours) of more traditional cooking.  Doing so in an oven (300 F, 180 C) is even better than a stove top, as there is less risk of scorching.  

A good quality recipe will give you a test to know when an item is done.  For example, meat falling off the bones in a braise.  Stock is one of the few things where its hard to give a specific test, but you should at least have good flavor development, and thinner bones will become flexible.

90 hours is simply unreasonable for almost any type of cooking whatsoever.  Perhaps they meant 9 hours, which is in the ballpark.

I suggest finding a better source recipe; if this recipe is far off on cooking time, how many other problems does it have?