Day trip to lake como from florence

Too Far. You will spend 8 hours trying to get there and back.

Will it be longer than going to cinque terra?
Florence to Milano central 1 hour, from there to lake como station 1 hour that is 2.5 hours trains and from bellagio ahead by ferry. Is it doable or sounding good only on paper?

The train to Milan takes longer than hour.
Cinque Terre is a lot easier.

You never hear about travelers taking a day trip from Florence to Lake Como.
Save it for another trip

Florence to Milan is almost 2 hrs. Milan to Varenna about an hour. And then you have to consider the connection times, etc. It's certainly doable but for me it wouldn't be worth the travel time.

The fastest trains from Florence to Varenna take 3:23. So if you left Florence at 8am and got back at 8pm, you would spend about 7 hours on trains and about 5 hours at Lake Como. Add waiting time and transit time for the ferries to the train time, and you won't be left with a lot of time to explore Varenna and Bellagio. To me that doesn't sound like an enjoyable day trip, but it's your vacation and ultimately your decision.

I agree too long a trip not worth it

With all the beautiful things that one could see in Tuscany within one hour or even less from Florence, you want to go all the way to the border with Switzerland?
Also, I wonder if you are visiting everything there is to see in Florence and surroundings?
For example, while you are in Florence, are you visiting the following:
Boboli Gardens with Pitti Palace and Fort Belvedere.
Stibbert Museum
Fiesole (on top of the hill) with the Roman amphitheater.
Piazzale Michelangelo and San Miniato basilica (most beautiful church in Florence and best view)
Medici Villa “La Pietraia” at the Castello district.
Villa Demidoff and park at Pratolino.
Giardino dei Semplici, Giardino Bardini, Giardino della Gherardesca, etc (see below)
http://www.theflorentine.net/lifestyle/2015/07/7-gardens-in-florence-that-you-have-to-see/
etc.

And I haven’t even gotten out of Florence city limits.

Thanks for all the guidance, very useful,