Venice & Florence

We spent 3.5 days in Florence and felt it was just right. I am big into Renaissance Art and so wanted to see.. everything.. and that gave us enough time and then some. I think it’s a great length for a first visit, not too long at all.

I cannot speak to Venice... yet ;)

If by 3 days you mean 3 full days, this would mean 4 nights in each. Is this what you mean? Or do you mean 3 nights, which is really 2 full days? Most people count a half day on arrival plus a morning on departure day as a “day”, but touring on departure day is challenging.

We spent 3 nights in Venice and felt it was enough time to see everything we wanted. We had two full days to explore, plus a half day on arrival day.

We spent two nights in Florence, and could have stayed another night. And we’re not big on museums.

You can “do” Venice in a couple of days - Doges Palace, the Basilica, the Piazza, Rialto, Bridge of Sighs, gondola ride, a church and a gallery yada yada.
The thing is that Venice has amazing cultural depth, and it takes time to discover it. The Querini, the Georges Cini foundation, Homo Faber if you are there in late September, the Biennale.

Spending only three days in Venice is not dissimilar to doing London or New York in a week or so.

I have planned 3 days in each. Is that too long?

No

3 days is not too long. But if I had 6 nights to split, I’d do 2 Venice, 4 Florence.

Good comments already posted.
I have been to Venice half a dozen times and enjoyed every day of it, but you can see the main sites in three full days. Florence deserves 4 full days, if you want to visit both the Academia and Uffizi. Also, a day trip to Sienna is good.

If you only have six days, I would do 2 full days in Venice and 4 in Florence.

A 3/3 split will give you a good sense of both. We spent 4 full days in Venice plus 1 day trip to Verona, there is always somewhere in Venice to wander. Florence wasn't as thrilling to me, but it wouldn't be hard to fill 3 days there.