Well the thing is you will always be stronger if you put more time into martial art. So let's start with the fact that by putting in limited time you will almost always be weaker than the people who put in more time than you. On the brightside most people on the planet Earth don't practice at all so there's that.
Fastest way would be to learn a modern weapons martial art. Modern weapons martial art tends to very straight forward in technique and can be quick to learn. After the learning you can keep practicing it to really master it. An example might be taking firearms course and what you learned at the range regularly. Initial lesson won't be in the thousands of dollars and you can put in practice as you see fit time and money wise.
Non-weapon art tends to require more training time to learn the techniques let alone master them. General rule of thumb in my opinion is older the style is longer it takes to learn the whole system.(once again I'm just talking about learning not mastering) Btw, the age of a style needs to be measured by the last time it really went through a growth and change. For example boxing is ancient, but it's constantly growing and changing in it's methods so this is a modern art.
Well that's just the general rule, and there are always exceptions to them. If you want to learn something quickly then learn boxing. And if you are in school learn wrestling with it because it's good training and it's free. If you are just looking to find a cheap/free information that you can look at and work at your pace... Well the attitude is all wrong to really learn a martial art then, but here: http://info.publicintelligence.net/USMC-... That's the martial arts that US Marines learns.
In most cases you need a good punch (which will be used pre-emptively) and the following skills:-
Threat & Awareness Evaluation
target hardening
The Fence
Coopers colour codes
verbal De-escalation
Physical De-escalation
become familiar with he Rituals of Violence.
Self Defence Law
To do this I suggest you read Dead or Alive The Definitive Self Protection handbook.
I have used the skills from this numerous times to stop situations ever getting to the point where violence became necessary.
Or if you are in the US I suggest The Little Black Book of Violence.
Its better to build a primo Black belt since you prime age of 18 years old with bodybuilding then getting into martial arts in age 21 because the supplements they didn't make you fell healthy at all.
Any true martial artist can train you to defend yourself in a week but you have to leave the sport crap away and train as it was intended
Having a relative or a friend who teaches martial arts teach you for free.
Learn how to run fast.
Yet again it depends on the instructor.
Effective, and not having to spend many years and thousands of dollars
