Francesco -
Congratulations on deciding to start RTK! I recently finished the book, and it's definitely worth the time it takes to complete.
I want to warn you - if you are not using the mnemonics, you are doing exactly what the author says NOT to do. You may be able to remember a few hundred kanji like this - but you'll lose them quickly. The first time I got this book, I thought that the stories were a waste of time, and only used some of the stories. Fairly soon, I got discouraged, and I gave up on the book. I only got up into #200 or so. Don't let this happen to you!
The mnemonics are the most important part of the book. Otherwise, you can just throw your book away and never use this site!
If you have trouble with understanding the stories, don't worry about it too much. Just make sure that you understand what the individual parts of the kanjis are (example: sun, tree, flesh, etc) - and you can make up your own stories if you like. That's what the study section of this site is for - we all make up our own.
If English is causing you trouble - are you able to understand another language more easily? RTK is also available in Spanish and French, if that might make things a bit easier for you.
Again, the mnemonics are important. If you are not going to use them, don't waste your time with the book.
To answer your most recent question as a personal example:
I've learned all 2042 kanji (but not perfectly - I have abut 700 in my "failed" stack, but I think I can get through those fairly quickly). I started this book in early September - so it took me about 5 months. But it could take you much, much longer than that - or shorter. If you have 30 minutes or an hour, (or more!) to devote to studying every day (instead of TV, for example), do it!
How much time do I spend on one story to remember it? Sometimes, only a minute. Sometimes, 3-4 minutes. Sometimes, I spend a long time, and even then I forget it. The best way is to spend as long as you need - don't rush. If you rush through it, you'll only have to come back and re-learn it, and that wastes even more time. It's best to concentrate on it right then. If you're getting tired or bored, close the book and do something else for a while.
Hope that helps - good luck!