How long should children be spending on their homework?

It varies. It depends on the age and the type of work to be done. Spread over a few nights in short sharp sessions should keep the child's interest high rather than several hours in one night or leaving it to the last minute. Guidance from the government (Direct.gov.uk) states:

Years 1 and 2: one hour per week
Years 3 and 4: 1.5 hours per week
Years 5 and 6: 30 minutes per day

Please speak to your child's teacher for further guidance.