So, I'm a huge computer geek! I mean huge! Hence, I build a new computer every three to four years. However I tend to do an upgrade(which consists of adding or changing the RAM, changing the video card and sometimes replacing the heat sink and overclocking it)every couple years. My current one is three years old and just got a partial upgrade of a new heat sink, case fans and got overclocked. I didn't have the cash on hand at the time for a new GPU or RAM. I'm hoping to be able complete the upgrade this winter.
I'm holding off on a new PC to see what Microsoft does with Windows 8 and 8.1. I thought they walked away too quickly from 7 so I'm hesitant to jump on 8. Plus I really hate it! As in hate like I want to set every copy of it ever made on fire and then burn the ashes for good measure!
As for smart phones I tend to replace them every two years or so. Basically when they drop dead or start acting funny and I have an upgrade available. I actually kinda of miss my first cellphone. It was a red Nokia that looked like a brick and only had a black and white screen. In other words it was a s dumb as you can get. I got it when I was in middle school and had it until Freshman year of college. That's about five years! I really miss how reliable it was.
As for when someone who doesn't clean their case biweekly should replace their PC. I tell people to try an upgrade of RAM first also moving to an off board video and sound cards can help performance. Failing that a clean install of the OS. That alone solves so many problems that people think cause them to need a new PC. I believe a computer usually must be replaced every five to seven years. In other words wait until your case starts to fall apart or some other catastrophe happens.
I had my first smart phone, a BlackBerry Curve, for three years and replaced it mainly do to the fact that I couldn't get any apps for it and the plastic coating on the keys was going as well as the coating on the cover as well. I really don't think the make phones as durable as the used to!
WARNING: Poster may be sarcastic reader discretion is strongly advised.