One of my friends was telling me recently on how his parents still uses this really old TV that looks like it is at least 7 years old. The interesting comment was how he mentioned that to him five years should be the cut off period to start replacing something for most items.
This is one of those things where depending on the situation not updating can either cost or save you more money. For recreational things, like a TV for personal use, I think you should generally keep it until it breaks down. I say that mainly because most people buy the latest and greatest without really taking full advantage of it anyways.
If the sole reason was for productivity though, then that is a different story. For example, just a few months ago I was at an office and the secretary was literally still using a typewriter and they had piles of documents everywhere. Like with that even though it isn’t broken, I’m sure investing in a computer would help a lot.
When you think about it, I guess this is just like investing in anything and whether or not the return is worth it as oppose to basing it solely on a want.