I recommend splitting your practice sessions into 4 parts and gain consistency and good habits by practicing the same way each day. Each part should be no more than 15 minutes a day.
Start with scales, arpeggios and chords (mandolin-family, guitar-family). Practice each scale with related modes, 2 octaves using all 4 fingers. Practice the arpeggios for each, 2 octaves. Finally practice related chords and chord progressions (mandolin-family and guitar-family only).
Next, work on any exercises, etudes or progressions as assigned by your teacher. Go slowly and be patient. Concentrate on proper form, posture and tone. Don't worry about getting through everything in one day!
Thirdly, work on any new pieces or material as assigned. Play it through first, or the part assigned, then work on each section slowly. Concentrate on tone. Always set your goal to play beautifully! Don't feel obligated to finish each day.
Finally, review past material, including old exercises and pieces you have already learned. In addition to review, sight reading each day is recommended! Open your book to a random page and read as best as you can.