I would say have 6 mos. of money to live off of minimally if you are just going to quit your day job and dive right in.  For most of us, doing the part-time gig is a great way to start.  You build your clientele until you can no longer cut it on nights and weekends.  And it is scary quitting the job for sure.   Be sure you have a contingency plan if something goes awry.   If a client doesn't pay you. 

 
I would also add to the list of things an entrepreneur should do is get a mentor.  Someone you admire that does what you do.  Great resource.