The short answer: 20 minutes a day, 6 days a week.
The long answer:
When you first start playing guitar, one of the biggest obstacles is the physical task of getting a good sound out of your instrument. Guitar is one of only a few instruments where you actually make contact with the note, not by way of a mechanical chain of events. So the mechanics need to be developed in your fingers instead, and your finger muscles usually need to get stronger, your fingertips tougher, and your coordination needs to develop.
This is why I do a lot of exercises with my beginning students, but also why I start them on one of their favorite songs in lesson one. I think the reasoning behind exercises is self explanatory, but favorite songs might not be so clear. Lots of teachers make you wade through scales, chords, dumb public domain songs, and note locations, before they teach you something you like, and without any critical thought about the original music and what the student’s current skill set is. I like to teach beginner students music they like right away, as long as the music can be approximated at their level, because they are so green that anything will help whether it’s an exercise or a song. And since people usually work longer and harder on things they like, songs usually win in the end.
Exercises, especially symmetrical ones, can be great because the patterns in them are usually a little more straightforward than the way that riffs and songs lay out on the guitar fretboard. Again, anything helps at this stage.
The thing with muscle development is that the student might be doing everything else correctly, but if they don’t have the strength yet, it still won’t sound good. It becomes a numbers game, with respect to time. They just need to keep using their muscles for them to develop.
20 minutes is not a lot of time, but that’s the point. Consistency is so important in the beginning stages that the quantity needs to be the minimum effective dose. Of course you can play longer if you want! But if 20 minutes isn’t completely easy for you, you probably shouldn’t be paying for lessons.