Well, the best way to go about it is to apprentice with an accomplished piano technician/rebuilder, join the piano technicians guild, take piano technology seminars and go to PTG conventions, go to piano technology schools, work in piano stores, read everything on tuning and tech work that you can... and basically eat/breathe/sleep/pianos.
I've been studying piano technology and been working full time in the industry for over 7 years. I paid my dues working on the floor as a top consultant of one of the largest and most prestigious piano houses in the world -ultimately resigning to start my own piano service company earlier this year.
Becoming a good piano technician is a serious undertaking - both from a time standpoint and financially. As for how long... like learning to play the piano... it never ends. A good bench mark to shoot for though is R.P.T. (Registered Piano Technician) status in the the PTG. It has taken me 7 years of industry experience and 3 years of constant study and tech work to prepare for my exams (the last of which I will be taking in July).
It isn't a job - it's a way of life. My suggestion if you are interested is to contact your local chapter of the PTG and inquiring about attending a meeting.
R.Cromwell