where do I go through customs?

You go through immigration in Paris, where they determine if you are allowed to enter, then stamp your passport. Customs is at Florence, where you pick up you luggage and walk out the "nothing to declare" door. That's customs.

Yes, sorry. I always get this confused about this. Thanks.

Customs is at Florence

That is assuming that you luggage is checked through to Florence - that it is not carryon and you don't have to pick it up in Paris and recheck it to Florence. In those cases, you would "go through" customs in Paris.

Yes, my bag is checked through to Florence. I was hoping I wouldn’t have to pick it up and check it again.

As long as it's all on one reservation, your bag should be checked through to Florence where you will retrieve it.

In terms of delays, it is immigration (in Paris) that is important. This is when they check and if necessary stamp your passport. 30-60 minutes wait is normal.
Customs (check on luggage contents) occurs after you pick up your bag at the final destination (Florence). It takes zero time as you just walk through the "nothing to declare" door without stopping.

I always get this confused about this. Thanks.

It's quite simple actually. Unless you have some purchases you wish to declare, customs is a walk-through. On your way TO your European destination, unless you've left CDG/ORY or bought from an airport store that is not duty free, you should have nothing to declare.