It is best to try and practice as regularly as possibly. It is far better to practice for twenty minutes every day than to spend four hours practising the night before your lesson. By practising a little each day you will have more chance of remembering what you have done before and also becoming more comfortable with the guitar. I would recommend that beginners try to practice for twenty minutes everyday. This should be twenty minutes of focused practice. Not sitting in front of the TV while practising or twenty minutes of messing around. Try breaking the practice session up into smaller chunks. For example, start out with some scales or exercises for five minutes, then move on to playing some chords for five minutes, then try practising chord changes and finally give yourself some time at the end to reward yourself by playing a song you like.