Minimum One Hour per day, Six days per week. If you have time to do more hours per day, and you don't overdo the exercises, feel free to do as much as is comfortable for you. More time spent on the program will produce more results. Just pay attention and stop if you feel yourself going into discomfort.
Yes, @jnaab, 30 to 40 minutes a day, 3 - 4 days per week is much better than none or less, but there is no denying that an hour or even two per day is going to move things along much faster.
Just casually singing along once in a while is no substitute for dedicated, focused, regular practice and study.
Quality is definitely better than quantity, but both quality AND quantity will produce steadier results.
More so than practice makes perfect; Perfect Practice makes perfect. There is no perfect, but you get the idea.