The Changing of the Guard takes place in a large square with some shops and restaurants surrounding it. If you can get there early, you can see how crowded it is and either stake out a place for the ceremony or browse the shops until the ceremony begins. In reality, you should be able to arrive at the last minute, at 11:45 or even 11:50. It takes place in a rather large square so there is usually plenty of room. However, when I was there in May there were busloads of Italian tourists that made it much more crowded than I had ever seen it before.
It only lasts about 5-10 minutes.
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