People think that singing is easy. Well, definitely it's so easy. However, knowing some different tips to sing better will take a lot of time for everyone. But if you have the passion and eagerness to learn, you can do it in a shorter time.
You may do research through internet for some good tips. Keep on practicing. You may set your prefer time, 30 mins a day is enough i guess. But it actually depends on your available time. keep in mind that there are lot of ways to improve your singing voice, and there are lot of rooms for learning techniques.