I personally head-cannon that it's been about a month since he started going to school and that we're only really seeing the highlights.
But if my tracking's correct, we've only explicitly been told 8 days have passed.
He was at school for 2 days
Took a 2-day weekend
Was at school for one more day
And then took a 3-day leave of absence.
He first showed up Thursday of last week and now it's the Friday of the following week. The hugging incident happened on Monday.
So, either Kissa is REALLY bad at temporal pacing or he's setting up a bit.
I'm hoping there's at some point going to be a line of dialogue or a page where Sou plainly states that he's known them for a decent amount of time OR in the service of a joke that he's known them for a ridiculously short amount of time.
It's nice to see that your math adds up to 8 school days too. And I hadn't thought to use the sperm donations as a two-week benchmark. I'm only remembering two off the dome which means we can confidently say AT LEAST two weeks to a month has passed. Assuming we're being shown every time he goes. Besides that, I think you're making a lot of assumptions about the timeframe of events that we can't really know or prove.