I'm confused. When reading a novel, I've encountered
It's around 3 days worth of stuff.
I thought "why not 3 days' worth of stuff?"
So I checked the Sketchengine, and got 2.2k results for days worth of and 1.2k for days' worth of.
In both cases, there were constructs such as "number days worth of noun".
As I understand, the first version is a noun string, when the second one basically says "this amount of work [belongs to / is done within] three days", so my guess is that both forms can be used, but the first one without the apostrophe is more reasonable.
Could you please verify my thoughts?