It's important to find a good fit for your music practice in your life. It should be challenging enough to promote growth, but not so demanding that you quickly burnout and are no longer inspired to play music. There are stories of legendary musicians having crazy practices routine's, like Charlie Parker's rumored 14-hour daily practice sessions, but for most of us, crazy practice sessions like that are unrealistic and actually not beneficial.
For most of us, the longest our brain can focus effectively on one point of study is between 90 and 120 minutes.
That's a maximum of two hours, and that's if we even find two hours worth of time to practice every day in our hectic lives. I'll be completely honest with you, I've been having trouble with even 30 minutes of focused practice every day. The best way to make sure you practice effectively every day is to be realistic with your practice.
Don't say you're going to practice an hour every day if you haven't thought through the logistics of what this practice regimen requires.
If you set an unrealistic goal for yourself, you'll quickly find that you get demotivated and "fall off the (practice) wagon." Make your goal for your practice regimen realistic. If you find that the longest you're regularly able to sit and focus on practicing your instrument is 15 minutes, three nights a week, maybe set a goal of playing 15 minutes for four nights a week and see how that goes. Maybe your schedule doesn't allow for four free nights of practice--try sticking to the original three nights a week but up your practice sessions to 15 minutes each.
Play around with how you plan for and structure your practices.
It helps to use a regularly updated calendar. I have found that for every practices session I plan, I need a half-hour of buffer time before and after the session. This means that if I plan to practice for 30 minutes, I need to find a block of time that is 90 minutes long in order to ensure that I have an effective 30-minute long practice session. The 30 minutes before my practice session, I take the time to set up my practice space, get all the necessary supplies I need, and approach the session with a focused mind so that I am not rushing through my exercises. The 30 minutes after my practice session, I take time to put my supplies away, clean up my space, and relax a little bit before going on with the rest of my day.
You don't necessarily need 30 minutes before and after your practice sessions to have effective practices, you just need to find out what structure works for you. Maybe you have a regular place that you practice at in your home, so you don't need to set up all your practice materials--they're already there. In this case, you'll probably need less time on each end of your session. Maybe you need to travel to your practice space and it takes you 30 minutes to get there. In this case, you'll probably need at least 45 minutes of buffer between driving to the practice space and setting up. Find out what works for you.
Write out your practice plan.
Your practices session will be most effective when you know exactly what you will be practicing and for how long. You need to define your practice so that you are able to notice your own success and growth as a musician when it happens. I like to write out the specific exercises I'll be working on, what to focus on in these exercises, and for how long I will be focused on each exercise. Then, I set a timer and after I complete the allotted time for each exercise, I check it off and move on to the next one. This keeps the practice session efficient and I'm never stuck just wasting time noodling around, thinking of what to practice next.