Salzburg – 2 or 3 days?

For me, three to four nights is ideal. It's easy to ride the bus out to Berchtesgaden, with the spectacularly beautiful lake, boat ride, and hikes. The salt mine is on the way.
A concert , perhaps at the Fortress, is always a good idea, along with the walk to the Augustiner Beer Hall. Salzburg is one of my favorite places.
Have a great trip.

Three days is definitely not too long there. We have been a couple of times and there is still so much more to see on future trips, to give you an idea.

And, the RedBull Museum is fun, and free. It's an easy bus ride.

Thanks all. Let me clarify. I'm not talking about time to visit sites (like Berchtesgaden, the salt mine or Red Bull Museum) outside Salzburg via day trips or on the way to/from the city. Just Salzburg proper. The places I mention in my OP. Thanks.

I think two nights is enough for those sights. The core area is pretty compact.

I had 5 full days in Salzburg, 2 of which were spent day-tripping to Werfen and Berchtesgaden, so 3 days actually in Salzburg, and honestly I could have spent longer.

Obviously it depends on your general pace when sightseeing, and how many things you want to fit in, but for me (and I like to be quite thorough, so I'm probably a little slower than many other travellers), 3 days was only just enough time to feel like I'd satisfyingly "done" Salzburg and seen what there is to see. So I would definitely urge on the side of 3 rather than 2 days, personally.

If you have the time, and the weather's good, I would recommend heading out to Schloss Hellbrunn, which is where we spent our final afternoon. It's a beautiful palace with incredible trick fountains, not like anywhere else I've ever been. But that's a "if you have time" sight, rather than an "absolute must-see" sight.

Thanks all for the good advice.