I am not the expert on this, i can only offer you advice on what i am doing. AFter H.S. i decided to work, i hated HS and didn't want to jump into a college. Well after some time i decided it was time for a change so i began to look into schools. I knew i didn't want to goto school for 4 years or goto county, although my county college is one of the best in the country. So i looked into technical schools. I found the Chubb (high tech institute) Institute of computer technologies. One year course. At the end i get a diploma and if i decide to i can finish the course with an associates online for additional money. 
You may say to yourself well i don't want a diploma i want a degree etc, and yes i do too but in this field your work speaks for itself, if your going into a managerial position for say an ad agency than maybe a business degree would help, but for the most part, doing logos, websites etc your digital portfolio is all that you have. 
Anyway i am 22 years old, been at this school for 3 weeks, first 2 weeks were basic composition, layout, understanding placement, looking at past methods of graphic design etc. This week are the basics of photoshop, than illustrator, than advanced photoshop/illustrator. The course also includes 3D Animation with 3D Studio Max and maya, flash, html and website building. Aswell as a whole host of programs. 
Honeslty i am not super impressed yet, i guess because i already know alot, but i hope to learn alot more when it comes to the 3D portion. 
As for how many years, it depends on the school, county colleges offer courses for 2 years or you can goto a highly acredited school for 4, depends on your area. Or find a technical school that cuts right to the chase, no bull**** courses like philosophy or math or science etc. 
We will see where this takes me, good luck in your journey and feel free to PM me for any other info.