Well ...
The XBox 360 has been on the market for 2.5 years and hasn't broken 20 Million units yet, and I think it will be facing tougher competition in the future. I expect that it will be replaced after 5 years and will have sold 35 Million units at that point in time; depending on how well it is pushed after the next system is released will determine whether it will pass 40 Million units or not.