I've noticed an issue with bar soap. When washing yourself at a water source with the soap present in your inventory, the soap will be consumed, but not completely. A pixel or so of the soap's "remaining" bar will never disappear, no matter how many times I use it; and even with such a small quantity present, I still receive the speed bonus for self-washing. I've been using the same one pixel of soap for about three in-game months now, and it's never consumed.
Unfortunately, I don't have any more concrete data to provide, only anecdotal data. I've washed both with the sliver of remaining soap, and without, and it is, indeed, significantly faster with the one-pixel soap sliver present. I've washed with minor body dirt, and with major, week-long body filthnastiness present. The last pixel of bar soap seems to be infinite, but remains effective.