the learning curve is pretty personal- some people are (naturals) and dive right in doing pretty good work- some people spend years and never feel they could quilt any but their own- also it can certainly take years to build enough steady business to be able to actually support yourself/pay your bills-
remember in many locations quilting is a 'seasonal' thing- many people do not sew in the summer time-they are busy doing other things- you may have no work all summer long- then be swamped from Halloween til Christmas---
if you are serious about really looking in to this first thing you should do is visit some dealers/ shows and try out a long arm- check them out and see if it's something you enjoy- or think you would enjoy- you need to remember- you are still dealing with customer's so----
also if you want to do this as a business to support your family (take the place of a full time job) you will be on your feet- working your arms, shoulders, back for many hours a day- you really will not have time to do your own quilts any more- you will spend all your time on other peoples quilts-
there are many many things you need to think about- and you need to build a business/customer base long before you quit your 'real job'---figure a couple years at the minimum...it is certainly not something where you can quit your job, set up a $20,000+ machine set up- and figure you will be making money/paying your bills within months...it just doesn't work that way.