When you ask your employer for permission for a second job (insofar as that is strictly legal), it's usually not a given that they ask for an estimate of the hours per week you will put in it. Instead they will ask you about the work to see if it might infringe on their business. It's a measure made to ensure that you don't get into their water, not one to protect you. That is why it's in your work contract and not part of the actual law.