A couple of years ago I was getting tired of being asked this exact question by people and more specifically people that wanted to be my students.
So I took a while, sat down and typed up what I call my " Boss Be Gone" plan. The timeline is 1 year.
In the end the key is " DEBT" . The reason you have a job is to pay your bills, right? If you didn't have bills you probably wouldn't work. So, the first thing is to look at your debt, do you have an actual working budget where you track your expenses in life? If you don't. DO ONE IMMEDIATELY.
Then figure out where you might cut back. Do you need the expensive car that you are making payments on? Could you do with a less " flashy" car? Do you need 400 TV channels on your cable?
If you sacrifice you can do this faster. Think about this.. if you and I both make 50K this year, but you have 40K of debt payments, and I have 5K, I now have 35K more than you in one single year. If you had an additional 35K this year, would that make you happy?
Let me leave you with this thought... Don't plan to replace your monthly income either. That is the wrong thinking in REI. Sometimes I get one deal every 6 months, but it is a doozy. So think about that you need say 40K to live every year. That is what you need to shoot for. Rather than saying I need 3,500 each month and shoot for just making 1 deal a month at 3,500k.
Primo is right in that you need to have some put aside before you cut the leash. But plan for it, be specific and it can come true, if you really want it to be true. To many people replace their job with another boss, for one where they are their own boss and you work more hours than you did before for less pay.
Good luck!