A little clarification is needed on my part.

As I understand it the difficulty of the investigation influences the modifiers on the skill roll as well as the overall time needed to succeed in the endeavour.

For every successfull test some hours are subtracted from the time you need to spend, every failed roll results in some kind of setback.

BUT: How many times are you allowed to make a test? Is it once a day? Is it once everey 12 hours?

Or does the number of hours you rolled after a successful test indicate the actual time you spent making inquieries? In which case the only thing you can influence is the quantity a rolls you have to make before accumulating the needed research time. But that seems king of boring.

Any thoughts?

You are right. There is no sensible way to interpret the investigation rules; they make no sense.

I tend to just make up what I think is a reasonable time for any given investigation task. I use the roll to adjudicate success or failure, rather than how long it takes. I've been known to use extended rolls, in which case I'll just require a roll once every defined period (again, using some time period I've decided on a case-by-case basis), and require some number of successes/accumulated degrees of success to complete the task.