Ken recommends practicing a minimum of one hour per day, five or six days a week. Do take one or possibly two days per week of rest.
You can go for more than an hour if you like, and your results will come sooner.
One of the BEST things you can do is to always have an inexpensive recorder going, and listen back a lot to what you are doing. You could get a very inexpensive cassette recorder at a second-hand store and that would be good enough. You can then listen to the sound of your voice, get over being overly-critical of yourself and move on to learning how to improve the things that could most easily be improved. The 3:1 schedule would be fine.
It is very important to make your practice time a PRIORITY in your schedule, or it will slip to become the thing you put off when time is tight. The more priority your practice has, the more consistent your practice will become, and in turn, the more consistent your vocal growth will become. If you can, then practice at about the same time each time you practice, and your voice will become accustomed to that "waking-up" at that time of the day or evening.
It is common for newer students to go through a time of some fatigue, because you are asking muscles that previously may not have been brought into use to do something substantial for an hour or so every time you practice. That fatigue will gradually diminish as you build musculature and stamina. That takes time. Eventually, if you practice diligently, you will grow far beyond that level of vocal strength.
So a more ideal practice schedule includes a recorder, and about 2 hours per day, 5 to 6 days per week.
Good Singing to You!