Given how famous it is, I was surprised at just how impressive Machu Picchu was. Don't just go to see it: take a little time and experience the atmosphere. We climbed up to the top of the peak at the back of the site, that's the peak you can see behind the ruins in all the standard photo's and it's well worth the time and effort. However, if you do this you generally miss the standard return trip down to the nearest town and have to stay an extra day.
Since you are however, only likely to be here just once in your life, it's time well spent.
(so in short, allow two days for an extended stay at the site of about 6 or so hours.) over 6 years ago