From what I remember in collage, a long time ago, lol.
The basics of wireless charging is that of a simple transformer. Two coils of different size, one is powered by the AC in the wall outlet which energizes the first coil creating a magnetic field which rises and collapses based on the Hz frequency of the power supplied, 60 times a second here in the USA for example.
The other coil is located in the back of the phone and is connected to the battery in the phone.
As the magnetic field rises and falls in polarity it induces an electrical current in the second coil in the back of the phone.
The ability of the second coil to convert and capture the induced current and ultimately charge the phones battery is a measure of its power factor. The power factor is at its greatest or best when both coils are as close as possible and, under ideal conditions, can be close to the same power factor found in a standard wired wall charger. As the two coils move apart or out of alignment the power factor decreases and the charging potential is lessened up to the point that the phone will not charge. So really there are three things to keep in mind if one wants the best wireless charging using induction, 1) keep the two coils in as close contact as possible, 2) keep the two coils parallel to each other and in alignment to each other 3) make sure the material located between the two coils " aka the charging mat and the phones battery" is conducive to magnetic flow "ie, not metallic".
If the above was TLDR, then the simple answer is, any phone case will lessen the wireless chargers potential, and depending on the cases material type and its thickness, some will effect more than others.
As far as saying how much would have to do with how strong the first coil is and how saturated the second coil is. This is a particular of the charging mat and the phone that it is paired with. So there is no hard and fast rule. There is only ideal and degrees of lessening ideal situations, not a good or bad.