How dirty can a sensor be and still be ok?

[ProfHankD]

Second, all other things being equal, at equivalent focal ratios (e.g., f/8 in MFT and f/16 in FF), the contrast of a dust spot is less in smaller sensors than in large sensors. (For the same cover thickness, the diameter of the shadow of a dust spot at f/8 is physically twice as larger as that of a dust spot at f/16, and the MFT sensor is physically half the linear size of a FF sensors.) So, dust spots in smaller sensors are larger but lower contrast, and IMHO are less noticeable.

Actually, sort-of the corollary to your second: smaller pixels make dust spots look larger, but with more diffuse edges, hence less obvious. I think that's really the #2 reason for MFT (with 4mm cover glass being #1).

Um, I don’t see how pixels have anything to do with it. Dust spots are typically on a medium scale, and so I think a dust spot will look about the same at 12 Mpx, 24 Mpx, and 61 Mpx, when an image is displayed at the same size. Am I missing something?

Dust that is significant is generally quite a bit larger than a pixel. Aside from having more pixels in the partially-shaded edge region for the shadow of each dust particle, there's also effectively more diffraction spreading the light at the smaller CoC for a smaller pixel and the microlenses are also smaller and hence more directional. Thus, for example, with all else equal, I'd expect dust to be more visible on 24MP FF than 24MP APS-C -- and that is my general experience; the dust gets higher-contrast edges. In other words, the ratio between the size of the dust and the pixel size changes with pixel size.

Does a 61MP FF show more dust than a 24MP FF when the images are scaled to 8MP? That depends on the interpolation and scaling algorithm. Certainly, the 24MP one is more likely to show localized color distortions due to the lower-frequency sampling of the Bayer pattern.