They always drop 10%, confirmed by Justin. So in theory you could drop 5 or more levels of the same skill if let's say it's 28.
28 - 10% = 25.2
25.2 -10% = 22.7
Also that being said, skills over 16 will almost always get a double drop as they drop 1.6
In my opinion, this is one of the irritatingly unrealistic things in BB.
If one season -(12 weeks?) tries to mimic one season in "real life" then 1 week in BB would be approximately 1 month in real life.
Do players´ skills really decline that fast in real life? Well, every player is an isolated case of course, but in general I don't think so, not even close in most cases. Personally I don't play on the level where I'd have players with skill levels, say 25 or 28, but it's even more radical (in percentages) when you consider a double drop (and possibly drops two weeks in a row as many have experienced!) from skill level 14 to 12 for instance. These kind of drops have of course been a regular occurrence in my BB history.
Also, while not being a double drop, my 36yo. player Sadinmaa just experienced single drops in BOTH IS and JR which again I find more or less unrealistic, and as a homegrown team who isn't "able" to buy himself out of skill decline trouble, also quite irritating. (I know and realize these haven't necessarily been drops of 1.0 skill levels on both skills, but from there I'll be able to lead the conversation to another point - I'd prefer having more accurate information about skill levels, but then again I don't see that being changed in BB any time soon).
Just as manager sõber mentioned, you don't see (or if you do, at least I've never seen someone writing about that) skill pops happening at the same pace, say from 14 to 18 in two weeks in one skill - which I understand and I think it´s actually a good thing, but I'm just saying skills probably shouldn't decline that fast especially when we reach these more mediocre skill levels (matter of taste, but maybe one could say 11-13?) and are not talking about extremely high ones such as 25-28.