well if you want a realistic answer.
provided you buy top-end hardware, every 5years you will *need* to buy a new system to play current games.
generally what you want to do is buy when the new consoles are released. as they will always set the benchmark curve for games, and most will then be ported between formats; so you know the scalability will be there for this reason.
graphics-wise though, new Shader generations are released every 18-24months.
i mean somethnig to remember is after hardware hits the market, it takes a few months for developers to catch up; and while yeah they develop in terms of "there will be more powerful hardware", there is no way to get a jump on new Shaders, or that Multi-Cores were going to be released, etc. Hardware that really breaks new ground in terms of what can be achieved. In that respect developers will always be behind.
So yeah, every 5years. Provided top-end purchase.