I play both guitar and sing(I'm a much better guitarist than singer) and have been devoting around 4 hours a day to both instruments. I'm noticing that I'm having trouble covering all the things I want to cover from practicing guitar scales, doing singing exercises, and improving and composing new songs. How have you set up your practice schedule to get the most bang for your buck?
There's an excellent book by Gerald Klickstein called The Musician's Way that is the best treatment of this topic I've come across. Klickstein is a guitarist as well, but the methodology he advocates is applicable to any instrumentalist
One thing Klickstein advocates that I have found particularly useful is about the tempos at which one practices. When one is working on new material, it should be practiced at a speed at which you make zero mistakes. Once you can execute a passage cleanly three times, you're ready to move on or increase the tempo.
He also encourages musicians to avoid mindless repetition. Make every note that comes out of your instrument as musical as possible, and always keep challenging yourself.