there really isnt any specific length of time thats most effective to everyone. more, its something that everyone has to learn for themself. for me, i dont really have a specific time that i practice, rather i just practice when i get the chance. this is directly related to how bored i am which is directly related to how much time i have.

my practices arnt even that organized, really. i just do what i think i have need to do. some days i feel that i really need to work on scales. some days i feel that i need to work my alt. picking, or my legatos, or my tapping. its sort of in terms of necessity.

though, something that is necesarry before some hardcore guitar practice... a hardcore guitar warmup. i usually get the metronome going and play along, gradually increasing speed of the metronome till its rediculously too much. by that time, im all warmed up, and my fingers have more accuracy and speed. warming up is important.