Student visa with 90 days restricted work permit

So, I have a Student visa with a restricted work permit, allowing me to work 90 full days or 180 half days. I was wondering if anyone knows HOW these days are calculated, and HOW STRICT this 90 regulation is. Is it calculated by the number of hours you work, considering 8-10 hrs a full day, or? Because I found a job offering shifts of only 7 hrs. Of course, if that is considered a full day, it makes more sense to search for a job offering a 10 hour day. This must be something they considered while making this law, but its all a little fuzzy. Thanks for your help in advance!!

As as student, and in general as anything else, you won't be allowed to work 10 hours a day. You can work 20 hours per week during the semester and a maximum of 40 hour per week during the semester breaks. This is a regulation which is strictly observed by companies which hire students as "werksutents".
There's previuos thread with a few posts on the hours that a student is allowed to work.

As a rule of thumb 20 hours per week counts as 5 half-days and 40 hours a week as 5 full days. All restrictions written on your documents are as strict as they can be in Germany.

Here you can find some more dicussions on student visas.