How long should I practice? You should read the previous blog about practice too. The amount of time you have available will depend on your circumstance (working hours, homework etc.) so you need to decide on a goal for your practice each week and work towards that. (This week I’ll learn how to play an F chord properly or I’ll learn a new scale)
If you have just started learning and have never played an instrument before then about 20mins a day should be fine (if you’re an adult). If you have a young child starting to learn then 5-10mins daily should be ok to start with, the important thing being to encourage them to practice and develop a routine.
If you are at an intermediate stage and want to make some real progress well then at least an hour a day should do it.
If you’re studying classical guitar for example and your preparing for a grade 8 exam I’d say 2 hours a day minimum.
After some time practicing you will get a feel for how long you need to get all your work done and as you start progressing you will feel the need to practice more.