Since time is precious and always lacking it makes sense to make the most out of your session. I have come to realise that duration is what matters. Playing the classical guitar is a life-long thing, and life is too short to reach perfection, at least for ordinary mortals like me. Once you realise that it does not matter that much how long it will take you to get to the desired point, but to get there indeed, the next thing is to determine your attitude. An hour focused study does miracles, and it is better than two hours of careless, aimless playing.
I begin with vertical exercises, then scales, legati, rasgueados, arpeggios focusing on tone (except for rasgueados). I try to study each of these sections with the utmost focus and attention. Then reading new stuff, and finally playing- studying the most part of my repertoire. Always read with metronome, especially when i start with new pieces. I divide my time in two parts: technique and pieces/studies, each part taking up half of the session.