I think it would be better if whetstone's effects were temporary. That waould be logical because the blade goes dull over time. Maybe make the effects dissapear after a number of battles or monsters killled?
Yep time limited modifiers make sense as sharpening only works for so long.
It'd be even better if it was a use count limit instead of time as its more realistic.
are you aware of time limited buffs on objects ? If there are none, this could be a big modding flaw :-(
AFAIK it is not possible to add time limited modifiers to items, though it may be simulated with item scripts somehow.