In the school term I try to practice about 2hours everyday on insturments im taking exams in(piano, clarinet, sax) and then about 30mins playing guitar and just kind of messing about on the piano making up little tunes!!!!!
In the hols I practice about nearly 4 hours most days!!!
You should always spend about 20mins on scales foreach instrument, and practice the hard ones 1st not the easy ones like Cmajor!! then spend about 15mins on each piece concentrating on the parts you are finding difficult. then depending on how much time you have, play other "fun" stuff!!!
Hope this helps in some way!!