Regardless of your level of guitar playing, their is one aspect of practice that remains constant...the length of your practice sessions is not nearly as important as the frequency of your practice sessions. Simply put, just 20 minutes of practice done every day can be more beneficial than 2 hours of practice done 3 times per week. The key lies in being able to positively identify your strengths and weaknesses. Once you know your strengths and weaknesses, you can use that information to set unique goals for each practice session. By narrowing down your practice session to 1 or 2 goals, you can actually be more productive in less time. You can use this strategy to create weekly or monthly cycles of practice sessions. Rather than trying to fit everything into one practice session everyday, spread everything out into shorter practice sessions throughout the week or even the month.
Finally, a word on how to handle your individual practice sessions. I like to divide my practice time into 3 categories; Warm-up, Needs Improvement, and Playing For Enjoyment. Since the length of practice session varies among guitar players, I advise my students to break the 3 categories into percentages. Your warm-up routine should be roughly 10% of your practice time, things that need improving should be roughly 50% of your practice time, and save the remaining 40% for enjoyment (after all, what is the point of all that practice if you can't enjoy playing guitar). Surely each category can be defined in many different ways depending on your specific goals for playing the guitar, but follow this general idea and your practice time productivity is sure to skyrocket.