I recently read about studies that show that with 10,000 hours of practice, most of us can become experts at almost anything including music. That equals about 20 hours a week for 10 years. I usually quote that number to my students and tell them to get started toward that goal.
As far as a practice schedule, I tell them that they should try to get some quality practice time in every day. It does less good to sit and stare at a guitar book for an hour every day than to have fifteen minutes of real practice a day.
There is a good website from Daniel Levitin, the author of "This is Your Brain On Music" where I got that 10,000 hours of practice figure. -
http://www.yourbrainonmusic.com/
I would be interested in hearing your insights on this subject