I am not without sources here.

I have gotten to know a few of my high school teachers years after i graduated. I have multiple friends I went to school with, an ex girlfriend who I talk to occasionally, and my friend's old roommate whom i used to hang out with who are currently teachers in the US. Also my sister is a guidance counselor at an elementary school (from previous redditing, I have been lead to believe this is a position not common in other countries, but she works at a school counseling kids and works teachers hours for teachers pay. Because of the age of children she deals with she does a fair bit of educating as well and must have lesson plans and the like).

The general consensus is that the first few years are difficult as you're adjusting to your job and creating a lesson plan that works. Once your lesson plan is set a few years in, it only needs moderate revision once a year, and from there on the only take home work is grading papers and reviewing notes from your lesson plan. This can all be accomplished in about 3-5 hours a week depending on the subject taught. If you teach high school you most likely have a study hall period where you can get work done, and a free period where you can get work done.

If you're only counting hours in the classroom teaching, all of my high school teachers taught 4.5 hours per day. Include study hall and free period you're up to 6. include lunch break you're up to 6.75, and time in-between periods getting you up to 7 hours a day.

for my 1365 hour calculation, i went with 7.5 hours a day as the average time in school seems to be slowly growing. But keep in mind that's including a paid lunch. If you went by my high school's hours and discounted the lunch period, the average teacher is only in "the office" for 6.25 hours per day, 182 days a year for a grand total of 1137.50 hours per year.

My friend who teaches math says he can accomplish all his grading and planning in his study hall and free period. My other friend who is an English teach says she takes home roughly an hour of work a night about 3 days a week, but she's still adjusting her lesson plan and will probably be doing less in 3 years time.

I firmly stand by my numbers as an accurate representation of schooling in my region of the US, seeing as i even padded my numbers in the teacher's favor.