Short answer:
There's no magic formula and everyone's needs are different.
Longer answer:
There are many ways to train but there are some trends in training that we can take advantage of.
First and foremost in voice training, we want to manage vocal strain. Vocal strain can result from pushing yourself too far very quickly, performing an exercise in a damaging way, over constricting your voice, simply training for too long, or any other number of things. How your voice reacts to what you do is unique, but there are flags you can watch out for that should shape your practice. If any part of what you're doing hurts or burns in any way, you should take a break or stop entirely and avoid doing whatever created the feeling in the future. If you feel those burns or pains consistently or even at the beginning of your practice, you should seek out myself or another voice trainer to help you find what's going wrong. If the next morning, your voice becomes raspy or the area feels very tender, you definitely overdid it the day before and you should scale back your training a bit to give your voice a rest.
If you're new to training, I've heard it said that shorter practice sessions will be far more beneficial than longer ones. I've personally found this to be true as I and those I've taught seem to have learned very quickly from sessions that are as short as possible yet very frequent throughout the day. If you're farther down the line and your voice is very stable, longer sessions are fine as you're likely tackling more complex concepts. Just make sure not to overdo it as many, many studies have proven you can only learn so much in a day.
Dysphoria management is another big factor we should be considering. Dysphoria is an incredibly strong motivator to cultivate a new voice, but forcing yourself to listen to your current one can be downright hellish and end up putting you off training entirely for long stretches. I personally took months-long breaks due to struggling with dysphoria while training, and so have many others. Of course, this is less of a problem if you're close to your ideal voice, and not a problem at all for the most part if you've already found it and are fine-tuning it.
Putting it together,
voice training seems to be best served in short but frequent bursts in training. For this form, try to keep sessions no longer than 10 minutes at a time, where 5 minutes is maybe more optimal. This could be something like: doing silent strength exercises while waiting in the grocery store line, singing along to a song on a short car ride, imitating someone from a podcast whose voice you admire (and kinda want to steal).
Try to get around three 5-10 minute sessions in a day, ideally spaced a few hours apart. It is absolutely ok to do a few more if you feel great fewer to none if you feel down. Your journey goes at your speed, and avoiding burnout is essential. We really want to make a habit out of doing these short sessions, so it can be incredibly helpful to anchor them to existing habits. A good example of this is to have a voice session around breakfast, lunch, and dinner. If you'd like more private times to train, maybe practice around brushing your teeth in the morning, getting home from school or work, then again when brushing your teeth at night.
The holy grail of this super brief habitual method is to tap into what is called the "default mode" of the brain. Put very simply, the default mode is the state your brain goes into, with all associated behaviors and habits, when you've got nothing to do for a moment. This happens essentially every time you're doing nothing while waiting on something. Instead of your usual habit, pulling out Reddit on your phone (guilty), spacing out, stimming and stressing, we want to try and make that voice training. Just imagine if we could idly voice train in any quiet moment without having to set reminders or cut out time for voice sessions! These would also be, by necessity, incredibly short sessions like the method described above that would avoid long exposure to voice strain, dysphoria, or just the normal frustration of failing at a new skill that is inevitable.
Oh yeah and whatever you do, have water on hand. Your throat and voice will thank you, and it's also just a great incentive to stay hydrated which is also awesome.
Happy training!