Florence airport customs

That is a small airport. It could take 30 minutes to an hour or more. It all depends on how many flights arrive at the same time as yours.

Where are you flying from?

It's not customs you have to worry about, it is immigration (passport check). Customs is the goods (luggage contents) check and is a random check only, most people aren't stopped.
You will only have to go through immigration if you are flying in from outside the Schengen Area, not if you are coming from another location in the Schengen Area. Immigration could take 30 minutes+.

It will not take long to get out of the airport. Your real problem is where to go, and what to do with your luggage, from when you get out of the airport until 4 pm. Most hotels will hold your luggage until check-in time.

I went through there in October last year. From landing to taxi stand. 50 minutes.

Where are flying from?

Claiming your baggage may be slow; the airport is tiny but efficiency could be improved. Most flights are internal or from Schengen states, so they have no immigration checks. Customs controls are very quick but there are sometimes a couple of agents doing spot checks. In most cases you can be out in half an hour.

Since it is likely that you are flying from an airport within the Schengen Area, there will be no passport control. Customs is just al walk through the “Nothing to declare” line. There are spot checks, but they are so rare, that I have never been stopped in about 40 trips to Europe or so.
From landing to taxi it shouldn’t take more than 30-40 min. Once on the taxi, the ride to the city center should be 15 to 25 min depending on traffic and location of the hotel.
Just because you might get to the hotel before 11am doesn’t mean you can’t check in. If rooms are available they will check you in. If not, they will check you in but you won’t have a room until later. In that case leave your luggage with the front desk and go for a walk and start exploring the city.