Not fast at all.
I highly doubt they were ever cleaned on my ~10year old 34401 and are not significantly (i diont how how polished these were to begin with) discoloured.
At least, its not much of a problem, when shorted with gold plated 4mm connectors (gold on CuO should be rather bad in term of thermal offsets) i measure below 1uV of offset, well below the 34401s uncertainty.
Wouldnt waste time and effort on cleaning these too often, only if you really need the highest level of precision.