15m is good for starters, but after a few months you'll need to get serious about practicing.
Not sure if serious??? Play for 15 minutes a day? Fuck - I WARM UP FOR AT LEAST AN HOUR!!!! That's just warming up: getting both hands and arms conditioned will take a minimum of two hours a day for six months for a newb.
A time frame for learning is contingent on many factors: Who is the teacher? What are their motivations? Ability of the student?
You need to be very careful with guitar teachers who work for music stores. Their only object is to get you to 1) Buy tabs/song books, 2) Upgrade your instrument 3) Keep you in the dark just enough so that you keep coming back but not too much or you won't pay tuition.
Bucket heads original guitar instructor - who is an obnoxious fat prick and can be found on Youtube - recommends a four and half hour a day warm up routine for all guitarists. That is not practical I know, but as a beginner until you develop some muscle memory at least an hour a day devoted to nothing but conditioning your arms and hands and building callouses is a must.
For a classical player , posture and form are critical as well so get versed in that ASAP! Fretting hand position is critical to master. You never want to have your thumb seen when playing classical guitar. It rarely ever leaves the middle of the neck and if it does, have a damn good reason and these are few and far between.
Some advice in general to any aspiring guitarist:
TABS ARE FOR FUCKING MORONS AND USUALLY WRITTEN BY FUCKING MORONS!
DO NOT BECOME DEPENDENT ON TABS TO LEARN SONGS! You box yourself in as a player and don't learn anything.
Learn your chords and develop muscle memory for all the major chords to where you can fret them with your eyes closed. After you master this, learn to pick arpeggios and continue building muscle memory with the same chords but now on both hands. Doing this you know where to find your chords by instinct and can also pick individual strings by instinct (or muscle memory). Once your major chords are mastered then go onto Barre chords, Power chords... later on learn minors, augmented, diminished , 9ths etc.
Once you can do the above move on to: Learning "chord shapes for lead playing". Example: A shape leads, F shape leads and D shape leads.... you will be able to smoke 80% of the guitarists you run into if you master these rather easy techniques.
Also - these lead shapes can be used in every type of music: Metal, Rock, Funk, Jazz, Blues, Country you name it. It's all about phrasing once you learn where to go to find them. There are only 12 notes so Phrasing is of paramount importance.
P.S. - find someone to jam with as soon as possible. Regardless of how bad you suck ass and are embarrassed to play in front of others... JUST DO IT! Usually you can find people who will be eager and more experienced than you to jam with easily off Craigslist.
