So often I hear from singers that they simply don't know how to use their practice time effectively. They in turn practice less or waste time during their practice sessions. Ideally, singers should aim for 60 minutes (or more) of practice a day but as little as 20-30 mins can still be effective if you use your time wisely. To help you out, I've devised a daily practice regimen to enable you to have better use of your time.
1) You should spend 1/3 of your practice time on vocalization. This includes breathing exercises (which you should absolutely be doing every single day), lip buzzing, sirens, scales, arpeggios, and exercises in flexibility. This is the most important area of your practice so do not skimp on this and go straight into the songs.
2) The second 1/3 of your practice time should be spent on your "homework." This would include any theory homework that your instructor has given you, or ear training and sight singing exercises. Transcribing your music into IPA (International Phonetic Alphabet) and other research activities is also an appropriate use of this time. Listening to recordings, researching characters, and discovering new music is an important part of your musical journey and absolutely counts towards your practice time.
3) The final 1/3 of your practice time is devoted to your music. It is important to note, however, that this is not just singing through your music. It is correcting past mistakes,exploring different styles and tonalities, and really working on diction. Having your music memorized does not equal it being learned. Keep improving until there is nothing left to improve.
One important note: your practicing does not need to be done all at once. If you only have 30 minutes a day to practice, take 10 minutes in the morning to do your vocalizing. Maybe after work, before dinner, you have 10 minutes to listen to some recordings. And ten minutes before your turn in for the night you can practice your music. Whatever you must do, be sure to PRACTICE!