We don't have a specific time each day for practicing, but the kids know it's one of their jobs, so when other opportunities arise throughout the day, I just remind them that they'll still need to find time to do their jobs. (Their other job is cleaning out the guinea pig cage!)
Obviously mornings are the best time to get it done, that way the day is open and they're free to play and have friends over. But we seem to still be able to fit it in later in the day, especially on these afternoons that have been getting hot. Practice time is the perfect opportunity to sit inside with the air-conditioner on!