How much effort should weekly homework be worth?
I use a rule of thumb that out-of-class work, including assigned reading, should be about twice the in-class hours.  So, a class with 2-1/2 hours of classroom work per week should have about five hours of outside work per week.

For a 15-hour load with 2-1/2 contact hours per three credit course, that works out to 37-1/2 hours a week, a fair approximation of full time.

For any class, some students will find the work to be easy and it'll take less time than you estimate.  Others will find it difficult and it will take much more time.  You're looking for the median and a fairly low standard deviation.  

Early in my teaching career, I asked some classes to keep a journal in which they recorded the time spent out of class.  The responses seemed highly suspect, so now I just talk to some of the better students from time to time and ask about how long they spend.