My husband and I visited Florence a couple of days ago. The statue of David was on the top of our list. It was awesome to see! The venue is not particular large so you could easily get round on an hour if you were short of time.
When we arrived in Florence, we checked online for tickets and we found one time slot available over the next three days, all other times were sold out. This created a real sense of urgency so we grabbed the tickets. Later on, we purchased a firenze card, and found that there were plenty of tickets available with our firenze card, we had unnecessarily bought an extra two tickets. I'm not sure why this happened. Perhaps there is a batch of tickets reserved for the firenze card or perhaps there was an issue with the gallery website which made it look as though the gallery was pretty much sold out for several days. Either way, if you do plan to purchase the firenze card, if time allows, i would recommend popping into one of the ticket offices to check if there are academia tickets available before you commit to buying tickets online.