We get a homework timetable each year, saying which nights homework will normally be set for each subject, and how long it should take. So for example, some subjects are set twice a week, at 20-30 minutes each, while others are set once a week at 40 minutes.
Occasionally teachers deviate from this, but if a pattern emerges that the homework timetable isn't being followed, parents are encouraged to contact the school and the school will look into it. There are also regular (annual I think) homework surveys, where for a couple of weeks pupils have to fill in when they get homework for each subject, and approx how long it takes. This helps identify subjects/teachers where too much or too little homework is given, or when it is regularly set on the 'wrong' day.
The other thing is that pupils' perception of homework varies - some think it is too much, others too little. That's why it is handy to know how much is expected for each subject - you might get a child who routinely takes too long, possibly because they are a perfectionist! This seems to be more of a girl thing! It is more than acceptable to sign the planner saying X has spend 30 mins on their homework but has not completed it.
Just checked on the school website, and for Yr 7s the average homework is supposed to be approx 4 hours 20 mins per week, rising to approx 10 hours per week by Yr11. Always has seemed fairly reasonable.