Scales are not always a favorite practice item.... many students aren't too excited about them. Why practice boring scales?
Ask any musician: Scales are essential--they are the building blocks of music. Thorough knowledge of your scales is comparable to fluency in reading or language. They are not the ABCs, but rather a much more integrated understanding of how words work together.
I suggest that ALL students spend 5 minutes per practice on scales. If you invest 5 minutes into scales every time you practice, you will be amazed at how much you can learn, and how quickly. The key is to ALWAYS practice your scales for those 5 minutes.
A complete beginner can spend 5 minutes per day working on the first 5 notes of the C scale. As the student progresses, the entire scale will be achievable. At this point, the student can also use the scale to practice other aspects of playing, such as tonguing more cleanly and slurring smoothly. The student can play the C scale in slow quarter notes, then play it again with two eighth notes per note, then with slurred quarter notes. The remainder of the 5 minutes of scale work is spent on learning a new scale, such as Bb.
Once the advancing beginner can play both C and Bb scales, the C scale is now quite familiar, and can be played perhaps only once at a somewhat faster (but even) tempo. The Bb scale is emphasized by a few repetitions (slurred, tongued, etc). There is now time to add arpeggios to the scales. In a few more weeks, as both scales and arpeggios are familiar, another new scale can be introduced. Every week or two the student will be able to add on either a new scale or new arpeggio, and as more scales become more familiar, they will take up less and less of the 5 minutes set aside for scale acquisition.
In this building block method, a student who practices regularly will be able to play all 12 major scales, one octave, with arpeggios, often within their first year of study.
Intermediate students who are able to play all 12 of their scales one octave can spend the remainder of their 5 minutes of scale study working on adding the second octaves to scales and arpeggios. They may also begin minor scale work.
Advancing students who are able to play all 12 scales two octaves and with arpeggios within five minutes can also work on minor scales, including all forms.
Please note that advanced students may spend significantly more time with their scales and arpeggios. They may be practicing 2 and 3-octave scales in major and all three forms of minor, and may use scales to practice a multitude of other aspects of playing, including double and single tonguing, consistency between registers, dynamics, rhythmic patterns, etc.