For a standard visit to Teatro La Fenice, you should plan for about 1 hour. This covers the main auditorium, the royal box, the Apollonian Halls, and the Maria Callas exhibition on the upper floor, with time to take in the gilded ceiling, the frescoes, and the tiered boxes that made this one of the most ornate opera houses in Europe.
If you're joining a guided tour, the walk-through itself runs around 45 minutes, but you'll often linger afterwards to revisit the auditorium or spend more time with the Callas exhibition, which pushes the total closer to 75 minutes.
For a shorter visit, 30 to 45 minutes is enough if your main interest is the auditorium itself. La Fenice is a compact site by Venetian standards, and unlike larger museums there's no real risk of running out of time mid-visit. That said, it's worth building in a few extra minutes at the end to stand in the auditorium after the guide moves on — the room is at its most memorable when it's quiet.