Speaking as a parent and a teacher here. I agree with the above poster, if by secondary school they need to be nagged then don't bother. The occasional reminder perhaps, or helping to negotiate time/space with competing siblings, but they should really be doing it themselves.
Primary school children may need help to organise themselves, but should be working towards independent learning and playing by the upper years.
Here, an hour each before school usually (in separate rooms so concurrently), as much as possible after school. One has had permission to miss non-essential lessons at school and come home to do more, but that was with music college auditions and major concerts looming.
The general rule of thumb is 10 minutes per grade each day, so for Grade 5, 50 minutes, but I don't really agree, it's the quality of the playing which counts. Playing a piece from start to finish is rarely, if ever, effective.
As a teacher, I've been known to 'sack' non-practisers, they are aware of this and generally are a motivated bunch.