What are the standard workloads for mathematics TAs at state and private universities in the US?
Not sure if this data point is really deserving of being an answer, but:

I graduated from a top-25 math department at a fairly large state university 5 years ago.  The standard TA assignment was 4 lower-division recitations (e.g. calculus) per semester.  Our contract permitted us to be assigned up to 20 hours of teaching duties per week, though most weeks the assigned duties took significantly fewer hours.  Students working as TAs for advanced courses (upper-division or graduate) were usually assigned 2 recitations, but were also expected to grade homework, etc.  

So what you describe doesn't sound totally out of line, unless you are spending considerably more hours per recitation.