This varies widely by league, so check with your local club. However, as the level of play and age of the players increases, so does the amount of time they practice. The youngest players (4-6 years old) may only practice once a week, though many will practice twice a week. Practices are usually 45-60 minutes long. Older recreational players often practice twice a week for an hour at a time. Players on competitive travel teams will often practice for 60-90 minutes 2-3 times a week. Players on the older and most competitive teams (Premiere and Regional teams) may practice four times a week.
The best thing to do is ask your child's coach whenever they are with a new one to find out the coach's practice expectations.