How long to stay in each place depends mostly on what your interests are. If you want to hike or kick back, 3 nights in the Cinque Terre is fine. Otherwise, 2N would be enough. If you just want to see the top 3-4 sights in Rome, 3N is enough. You're first day is pretty much a non-day if you're flying in from the US for example - little sleep, jetlagged, basically zonked. That gives you 2 full days when you may still have lingering haziness. You can see the highlights of Florence in one day (day trip from Siena). If you want to explore and enjoy Renaissance painting, sculpture and architecture, 3 full days will barely be enough (4N). 2N in Venice gives you barely a day and a half there - really not enough.
The most efficient route would be to go to the CT from Rome by train, then Siena/Florence and lastly to Venice.