What is the average teaching load for a math postdoc position in the US? What is the average teaching load for a math postdoc position in the US? And can someone please explain this in reference to the trimester or quarter system, which I'm not too familiar with.

According to most job ads I've read, it seems for universities on trimesters, it's 3 courses per year, and for universities on quarters it's 4 courses a year.

However, is it the case that the summer term (in either system), is different from the other terms and that teaching usually doesn't occur in summer? So it's 2/1 courses per term under a trimester system and 2/1/1 courses per term under a quarter system?

Usually how many weeks are there in a term under each system? And usually, how many hours per week is there in a normal course?

Obviously, I know these answers will vary from university to university, I'm just looking for a ballpark answer.
The general answer isn't really a number. I expect that in most cases a teaching load is similar to that of a regular faculty member, though that isn't necessarily the case. 

The semester, trimester, quarter system defines how many regular terms there are per year, with trimester and quarter often being synonymous. It is the number of terms that most students attend in a given year. Two semesters make a year on the semester system, etc. 

A post doc is more likely at a research institution than a liberal arts college and the teaching load differs. In some R1 universities the teaching load is one course per term. At Dartmouth (quarter system, "Dartmouth System"), the load was four courses per year, or one per "quarter", though you didn't need to spread them out over the four quarters. I don't know if the system has changed and my knowledge is very old. The summer term was just like any other and students were expected to do something academic, but perhaps academic related travel. 

At some State universities, the load is two courses per term, so four in a year on the semester or six (shorter courses) per year on a trimester system. Again, it might not be necessary to spread them out evenly and it might be possible to "buy" yourself out of a course or two using grant funds. The load might be lighter than that at some places, but not likely heavier at an R1.

At most R1 sorts of places if you have more than one course to teach in a term it is more likely to be two sections of the same course than two different courses, but not guaranteed. At some places the tenured faculty (and probably post docs) teach in their specialty in advanced course, but, again, not necessarily. 

At a liberal arts college the load might be four or even five courses per term. But there are few post docs at such places. 

A post doc might possibly not have teaching duties at all if the are paid out of grant monies. It would be uncommon, I think, to have a higher load than the regular faculty. In some situations you might actually have to beg to get teaching duties at all.