For the true young beginner just starting to play the horn, I recommend only 5-10 minutes of practice a day for the first few months. Older beginner students will profit from 10-20 minutes of practice per day.
After a few months, beginners can gradually build up a repertoire of simple exercises, solos and etudes to practice regularly, so 10-20 minutes a day is a better goal. After the first year of playing students can practice 15-40 minutes a day, depending on their age. More advanced horn players may practice between 45 minutes and several hours a day. Longer practice times are most efficient when divided into several sessions.
Expectations need to align with the experience of the player: Playing a musical instrument involves so many different parts of your brain! Practicing is not only playing and having fun: it's also hard work. Be patient and celebrate each little step. Remember that in the beginning, the brand new player is akin to an infant, learning to roll over, sit up, crawl, etc. Walking will come soon enough....
In addition, I ask students to try their best to practice every day of the week. Daily (or nearly daily) practice is essential for building beautiful tone and lasting endurance. If you plan to practice 7 days a week, when "life happens," we usually still manage to get five good days of practice per week. Of course there are weeks when that doesn't happen due to holidays, illness, scheduling conflicts, etc., but trying to practice every day often gives us the ability to average five good practice days each week.