This article is from the Classical Guitar FAQ, by Joshua Weage (jpweage@mtu.edu) with numerous contributions by others.
Practice as much as you can, but make sure that when you practice you
practice properly, and don't just play around. Playing around is fun,
but you will make much faster progress and have more fun in the end if
you learn to distinguish it from serious practice (which is not really
fun).
A couple of other quick points: Don't over do the practise so that when
you say to yourself 'Right, time to go and practise' you don't give an
inner-groan and the prospect of more dull playing time. Don't do so much you
get bored with it. You practise so you can play and have fun and enjoy it.
Also, if either of your hands or fingers do begin to hurt - rest. Little to no
benefit is gained by playing whilst in pain. You can build stamina by
playing after resting!
 
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