Kind of depends. In this part of Italy the greeting "Buongiorno" seems to turn into "Buonasera" after or even during) the long Pranzo (Mid-day lunch) which starts around 12:30 and can last until 3pm or 4pm Most shops that close for lunch re-open about 4pm although more and more Big Chain stores are open all day nowadays. Buon Mattina or Buon Pommerigio is seldom used here at least. It's anything after about 6pm in England I guess.
It's not about time, it's about daylight, evening would begin around dusk which would depend upon time of the year