Music teachers, parents, and directors all instruct students to "Practice!". Sometimes a specific amount of time is advised: 15 minutes, 30, or even 60 minutes a day.
What do I recommend? More than duration, beginning students just need to develop a habit of getting out their instrument on a daily basis. Once it is out, chances are the student will want to play a bit. This is where parents can be a BIG help: helping find one (or two short) sessions at the same time, in the same place daily. Guiding and coaching students at home creates more success and progress in their music studies.
In asking students to bring simple notebooks, I have the opportunity as their music teacher to specify what and how they should practice. If often have them recall to me what we discussed, and create bullet points from this. If I find that the student is not practicing (always apparent in the next lesson), I will suggest a time frame, or have them check boxes in their lesson books or notebooks.
Above all, students should be patient and practice faithfully. As with any skill, correct practice will lead to proficiency. Everyone has their own learning curve. Being able to handle Fur Elise on piano, or Mozart's Clarinet Concerto after a year of study is pretty much unrealistic.
Good foundations are important, and the first is to make practice a habit!
Now, go practice!