The exact time when the drop cap reset is not known. The only thing we know is that it is resets weekly.
Q: How do I earn decorated weapons by playing the game?
drops are awarded for play time over the course of a week. As your
play time increases, the drop rate will slow down. The drop rate will
reset when the week rolls over.
from the Arms Deal Update FAQ
There is a website that shows a timer of the cs:go cap reset. I am not sure if that is accurate because I searched for the exact time using Google an no one knows it. Also, other players said that they didn't get any drops after the timer ran out.
Someone on HLTV wrote:
its all depends on the first time u get ur first drop ever
from HLTV Forum
It could be true but it is not very trustworthy...
We do not know the exact time when the cap is reset.
My guess is that the week starts with the Arms Deal Updated. That means the first day of the drop cap week is Tuesday (the updated was released on 13. AUG 2013). I did not verify that but it would fit together with my drop rate.