Hey Rawonall. I have news on this, and sorry for the drop-off. I'll answer as best I can.
The basic way that the matchmaker works right now, is that it starts by looking for two people with very similar MMR. The acceptable MMR difference widens over time, basically each time that glowy-thing in the top left corner pulses. We have a number in place where the game decides that if two players' MMR is over this number, it would be worse for them to be paired together, and better to play a repeat opponent. We're talking about when you end up nearly getting matched with "God" when you're a tiny bug for example, that kind of difference. This limit and preference for a repeat opponent is meant to start when about ten of these pulses have taken place. Currently the game is doing that sooner in some cases, and was not intended. It will be corrected with the next patch that is coming out next week. (You'll hear about that in the next Weekly Post)
There is no set cooldown time, as each pulse is slightly random in length, but if you count the pulses and average them you'll get a good understanding of when it starts being more likely you will get a repeat opponent. Again, about ten pulses in the upper-left corner.
I hope this answer helped!