1 day — Not enough, but...
Uffizi (morning), Duomo area + Ponte Vecchio (midday), Accademia/David (afternoon), San Miniato sunset. Brutal but possible if Florence is a stop on a larger trip.
2 days — The minimum
Day 1: Uffizi + Centro Storico. Day 2: Accademia + Oltrarno artisan workshops + San Miniato. Two days covers the essential masterpieces with time to breathe.
3 days — Ideal
Add: Duomo dome climb, Pitti Palace + Boboli Gardens, a day trip to Siena or Fiesole. Three days lets you actually enjoy Florence rather than sprint through it.
4+ days — Luxurious
Add: Chianti wine day trip, a cooking class, Bargello Museum, Brancacci Chapel, the Oltrarno in depth. Florence + Tuscany countryside becomes a complete experience.
Our recommendation: 2-3 days ideal
The Uffizi and Accademia need advance booking. The Duomo dome climb is worth the 463 steps.