What does the stability of the nanomaterial mean exactly when considering the different possible environmental compartments? Are hours/days - weeks/months not too short?

The stability for the environmental compartments has the same significance as for health. It is certainly sensible to use the same classification for the biotic part of the environment as for human health. For the abiotic environment the given time frames are if anything short; however, in the context of precaution this is advantageous.