Lake Como

Hard to say 5 nights? The main thing is just to enjoy being there. It is a wonderful place to just slow down and enjoy it. There are the ferrys to ride, the bus trip to Switzerland, towns to visit, Gelato to eat, the walk along the lake in Varenna.
Enjoy.
ED

Ed right, it is a good place just to relax and take your time riding the ferry and enjoying good food and wine. I would assume you arrive in the afternoon and leave in the morning so you will really only have 3 full days. Personally I think that would be ideal. You must visit Bellagio, it's just beautiful.

What they both mean is that there is nothing to do there as compared to other Italian cities. If you are a go-go person, shorten your time there. If you want to relax, stay a while. It seems as though the ages of travellers there are skewed a lit bit older.

If you're go-go, it's definitely a looooong time to be in Como. If you're looking to get away, and relax, great plan! :)

maura,

Whether 5 nights is too long depends to a large extent on what you want to do? As the others have suggested, Lago di Como is a great place to enjoy il dolce far niente and while 5 nights just for "relaxing" may be too long, there are other choices.

If you're planning to explore other parts of the lake, then 5 days would probably be just about right. You could take day trips to Bellagio, Mennagio or some of the elegant Villas along the lake (perhaps Clooney will invite you for lunch?). You could also take a day or so exploring Varenna and visiting the gardens of Villa Cipressi or Villa Monastero, the Fiumelatte (waterfall) above town (not very spectacular when it's not running, but the hike was nice). As I recall, there's a Falconry or something above town that can also be visited.

Use at least one day to enjoy Varenna, sample the local Gelato, enjoy Laura's Ravioli at Albergo Olivedo, walk the promenade that goes along the lake, etc. There's lots to keep busy with if you're so inclined!

Happy travels!