I haven't been to Venice. At least 2 nights in Siena. More if you plan any daytrips. At least 4 nights in Florence, more if you plan daytrips. CInque Terre, depends on time of year. At least 2 nights, to me it depends on how many trails would be open. Lake Como, at least 3 nights. The thing with Italy is that, in general, it's so small, that if you find you've budgeted too many days to a city, there always is some kind of day trip that can be done.
It is a very difficult question to answer without information on season. We spent 3 nights on Lake Como and it was too much, since it rained for 2 days solid (late October). Similar with CT.