Does it really take 6 minutes and 20 seconds to properly fade-out a single precision audio signal?
You start off with wrong assumptions, so your result is nonsensical.


Your definition of good and bad should be based on some psychoacoustic understanding of what type if distortion and artifact is actually audible and/or objectionable
The 24-bit mantissa argument (it if were valid at all) would only hold a fixed point number. In floating point you can apply pretty much any gain you want. The smallest non-zero gain is actually 1.1755e-38.


There are various different ways to design a fade: linear, quasi-log, "half-window" functions, all kinds of continous derivative functions etc. They all sound slightly different and the best choice really depends on your application: musical fade, cross-fade between two signals, emergency fade out that needs to be fast, etc.