A significant proportion of infected need hospitalization to survive. If we all get sick at once, there simply aren't enough hospital beds to go around, not to mention other supplies. If the hospitals go far beyond capacity, the fatality rate will be _far_ higher than otherwise.
As to how long, I haven't seen good models. In the US we don't have very good data either, because we're testing basically noone.
Some of these models are also different in nature, which adds to the uncertainty: some are epidemiological models, some are economical (used to evaluate the impact of the measures on the long term).
Lastly, a lot of qualified people speak and try to give estimates. I'd love if they don't, because currently the best answer is "we do not know yet".
That said, I think that going over two months of lockdown will be probably ruinous for the country as whole.
And I'm not sure if people can hold psychologically being locked up for one year.
The question is, for how long? Until the infection curve goes down? Until there is not a single infected cases in the whole country?
While you can't know when this will happen (especially the former, most models are inaccurate because too little is still known about this virus), I don't believe it is sustainable to keep an extended lockdown on a whole country (as opposed to "just" Hubei, which however has almost the entire population of Italy) more than a few months.