My parents stayed in Sander's apartment for a total of 6 weeks. They tend to be extremely picky and difficult to satisfy, but they absolutely loved the place, and within a few days had asked whether we could rent it long-term (unfortunately for them and fortunately for you, that wasn't an option).

Sander's apartment is beautiful, clean, well-decorated, in a perfect location, with a beautiful view. Sander was helpful from the moment we booked (he let us stop by and see the place in advance) to the moment they checked out (including making arrangements -- from Spain where he was traveling at the time -- to ensure that my dad was able to get back in when he locked himself out). I highly recommend Sander's apartment if you're looking for a great place to stay in Amsterdam.