Are you about to come to Amsterdam for a very limited time but still want to rent a room? Rooms are usually rented out per month, but it is possible to find rooms that are rented out for shorter periods.
The important thing to remember is that most people who want to rent for short periods do so during holiday seasons, especially during summer. Thus it is imperative that you are out well in time.
Already now in June it is very late to start looking for a short-term rental during summer. That said, it is not too late, but you need to start now if you want a room during July or August.