The goal of practice is two fold :

Goal 1. To make singing an intrinsic part of your physical behavior, almost like a natural reflex, effortless and easy.

Goal 2. To constantly introduce new challenges  and improvements to the voice .

What is required to achieve both is Practicing with Mindfulness. 

The ideal 10 minute practice session 

The practice duration could be as little as 10 minutes  for a beginner everyday but the important part of the practice is to orient oneself and focus on what the goal is for that particular practice session. Paying attention to your body and the protocol of the exercise while you are practicing is way more important for a singer to benefit from Practice. Focussing on the physical sensation of sound production : where we feel resonance, where is the sound coming from, where is it going,  where does it feel tight, where am I stressing etc. 

The ideal 10 minute session is the minimum and most effective  way to practice for the purpose of vocal improvement and learning new techniques. 

Quality Vs Quantity

Duration can vary as long as long as the practice session is done with complete focus and attention. It is also essential to be mindful of your vocal fitness and tailor the duration around it to avoid injury. 

The importance of the Daily session

Consistency is paramount. Daily practice is essential to remain connected with your music and your practice. The discipline of daily practice keeps building vocal strength, stamina and ability on a daily basis .  The duration is not as important as the consistency . The motivation remains high with daily practice.