Starting from scratch with very little knowledge of .NET, how much ASP.NET should I learn before I start to dive in ASP.NET MVC?
As much as you possibly can.
The more you know and understand the more effective you will be.
Actually, I would posit that you should learn very little traditional ASP.NET webforms, or not at all.
ASP.NET MVC uses a totally different paradigm than "traditional" ASP.NET webforms, and unlearning part of it is almost a requisite. The core engine of ASP.NET webforms -- the Page Lifecycle and the Viewstate -- are both absent in ASP.NET MVC.
This is especially true if you come from a background that is more akin to MVC, like PHP, or Ruby on Rails.
Buy a simple book and go thorugh the asp.net mvc nerddinner example of scottgu. It will cover all the aspect of the real application.
here is the link: http://weblogs.asp.net/scottgu/archive/2009/04/28/free-asp-net-mvc-nerddinner-tutorial-now-in-html.aspx
and here is the real site: www.nerddinner.com
Around -100% of the webforms should be enough, considering webforms are more about having things automatically handled for you and ASP.NET MVC means A LOT more control over most of the things :)
Ideally you should learn MVC first as it forces you to learn and understand the various parts of the web such as HTTP Requests, the statelessness of the web, HTML, CSS, etc.
As a result, if you ever decide to use webforms, or are forced to use webforms, you understand how things are abstracted away and can better code to the efficiencies of the application.
The appeal of drag and drop from webforms might be very very appealing at first. But after a little time, you can see how nice MVC is for the fact that it forces you to take total control of your code from the get go.
Personally, I dabbled in webforms for a little bit -- but after using MVC, there's no going back. And on a side not, MVC is built on top of webforms, so you can always use them if you want to.
As much as you can. ASP.NET MVC is only part of ASP.NET - so for building scalable and fast ASP.NET MVC applications you actually must learn "all" ASP.NET (I think you must learn Webforms too - at least partially)
I think it depends on your programming background. If you are already familiar with web development and its stateless nature, then by all means learn MVC first. It is much more "pure" web development. However, it requires a much more in-depth understanding of how the web works.
On the other hand, if you are used to developing in a stateful environment, it can be much more comfortable to learn Web Forms first. If you are trying to learn a new language/platform, adding the extra trauma of learning to work with a stateless environment is unnecessary and may slow your progress.
Ultimately, it comes down to what you are comfortable with. Read up on both, and see which makes more sense for yourself personally, and for the task at hand. Neither Web Forms nor MVC are the end-all be-all of web development. They are both tools, and as with all tools, each is appropriate for different tasks.