I think this is a very individual question. how much some one does and should practice depends on factors like availible time, experience and how strong you hands already are. Ultimately the best way to practice , is push your boundries and focus on what you are not comfortable with the most.
I come from a guitar background and years as a 'shredder' I spent many years paracticing and play all day every day, until my illness made it so I couldnt do that. i had a hardecore sheduale of endlessly pracicing scales and finger exersises etc etc. its all ingraned into me now where its realy not nessesary I do that with the uke, i just work on songs. finger strumming is still a challenge so I focus on that a lot, doing rolls and such.
But for beginners I would recomend a schedule something like this not too long on each to prevent injury, 5 -10 minutes , taking little breaks between each- severe discomfort or pain-stop! stretch and take a long break.
stretching fingers, hands, neck, back and arms (you realy dont realise how tention in other areas can so affect ones hands)always good to start and finish playing with streching and massage
some easy chords, then challenge yourself with some you dont know from a chord chart or book, keeping track of the extra hard ones.
strum patterns and finger picking, work in a metronome to these exersizes and the the next...
scales, practicing alternating beteen differnt notes such as quarter and eight notes etc
learn some new songs
just listening to music. listen to the cadence, what beats are being played on, try to identify chords by ear, hum/sing melodies, how the various instruments (voice is an instrument too) are interacting.
hope this helps