This is a question most of us think of, even further down the track of our musical careers!
A lot of numbers are thrown around. A common statement is 30 minutes a day minimum. This is great in a perfect world with a vacuum absent of any of life's distractions. In the real world we have things that come up. For kids, there's homework, extra curricular activities, other things they like doing. For adults there are all the chores we need to do, excercise, looking after kids or pets or ourselves! Not to mention other activities we like doing too.
My view on practice is different. When first starting it is important to try to set a regular time that you could possibly pickup the guitar every 2nd day. Be it after dinner or before school. This initial period will last a month or so, to ensure a habit has a chance to set in. This gives us a chance to steadily improve as well and give us a few tricks under our belt to make us feel like there are some things we can do that arent a massive chore. This should ideally be rather short sessions, 10 to 15 minutes.
After the initial period we can feel better about the guitar. We can see improvements, we have a few chords that we can reliably play and a few songs we feel good about. Now if you really focus on the guitar, what you are doing, the fact you are living what you have wanted to do and pat yourself on the back for starting. You can also say to yourself "I am a guitarist"
Now what does a guitarist do? They PLAY guitar!
Not practice but PLAY! This is your instrument now! after a month you will have songs you can play, Riffs you can jam out
From now we no longer say "how long should I practice" we say "What should I PLAY"
Once we shift from a time based mindset to a task based mindset we can make real progress.
Your Task can be "I have this song I want to play" and you will play through the song. Afterwards you should reflect. How did it go? what parts were hard? what parts should be my next task?
The amount of time you practice is irrelevant. The tasks you complete, those are how you should look at playing guitar between lessons.
Till next time, keep PLAYING!