I already had a stange feeling for a few days, and now I took the time to verify it.
Inspiration gain will go down the more you already have (in realtion to your max inspiration).
I can't say if its a curve, but it looks like it, at half (of max) inspiration, my chars gain only a fraction of what they should get per second.
Going lower (using up inspiration) will get its speed back to normal.
I hope that it's a bug, cause I won't be able to use up all my inspiration 3 times a days to get the most out of it.
As a side note, the inspiration-mechanic in general is not efficient, JC said he wants people to be able to take a few weeks break from salem and come back without much penalties.
Max inspiration being filled up in 3.3 days is exactly the opposite, it should NOT be related to how much % are already filled up, and be at least 1 week of inspiration that can be stored on a character.