Clearly it is a bad idea to buy a property

a C2 is not and never could be habitable..   If you put in a bathroom and connect it to the sewer without the relevant pratica, that's a big problem.  If you dont have enough windows to ensure aeration and light, as well as other stuff,  you cant change the destination to a house.  Its true, nobody can stop you sleeping in your garage, but its not a house.  You certainly can never rent it out.   When you decide to sell, its still a C2 and can only be sold as such.    Living in a C2 does mean fines and sanctions if they catch you.     If its part of a condominium they can stop you living in it, but they cant stop you doing the necessary work to make it habitable and doing a change of destination.  Obviously if its only 2 metres high you can never make it a habitable property legally.    If you were to apply for a visa to stay here longer than is currently allowed, the address would not be acceptable.    Alot depends on where and what your C/2 is - the neighbours could make life very difficult. 

Youre trying to split hairs.  Its a C2 you cant live in it, either for a week or a year.  Your question is how long could you get away without anyone noticing that you are living in a magazzino.   That depends on your neighbours or if you get stopped by the police.

Is it already an apartment that you want to buy that is a C2, or is it a magazzino that you would turn into a holiday home? 

Not trying to split hairs, trying to clarify what the rules are. If you can't live (personally I would use the word 'stay' rather than 'live') in it for a week, presumably you can't live in it for a 24 hour period either, in which case the statement  'no one can stop you sleeping in a C/2' cannot be correct.

I have seen people on youtube renovate C/2s (old barns etc) in the Italian Alps and stay there for a limited amount of time, long weekend etc. I just wondered if there are formal limits to how long you can stay in a C/2 or it just depends on whether the authorities remain unaware of what is going on or choose to turn a blind eye.

Using a property categorized as C/2 (warehouse/storage) as a holiday home can be a complex issue, as it involves local zoning laws and regulations that vary by location. In general, properties are assigned a cadastral category based on their intended use, and using them for a different purpose might require a change in classification or special permissions.