I know people that are still using Titaniums and 500MHz iBooks!
It depends on what you want to use the system for. People that use a computer for simple tasks like checking e-mail and very basic web searches have no problems with old systems. Why should they upgrade to something costing typically over a thousand dollars if all they want is basic functionality?
Another thing I've found extraordinarily interesting is how Apple can change some of the interface components, and some people don't even notice it. I was on the phone with a customer (a Google employee, believe it or not) and she didn't know how to determine what OS was on her PPC based iMac vs. her newer MacBook. I asked her what OS she was running on the iMac and she didn't know. I asked her what the dock looked like (reflecting tray vs. icons on a solid background) and only then did she realize that the appearance of the two was different ... and that's presumably after she had been using the two different systems for years. The iMac was running Tiger, the MacBook was running Snow Leopard.
How often should you update it? Update it when you research the topic and think the changes are worth doing. A lot of people really hate Lion and Mountain Lion. I don't have a problem with either, but on the other hand I have no problems at all running a Snow Leopard or Leopard system. As for Tiger and its predecessor, IMHO, forget it!
Regarding Windoze, have your tried Windows 8? I tried it at a local office store, and maybe it was "limited" for store usage, but it seemed like a really obscure operating system to me. I've never liked Windoze but that thing seemed to be off on a tangent all by itself. It wasn't, IMHO, even remotely intuitive. Quite strange, really. I have to wonder if some people won't upgrade their hardware on those systems because they don't want to commit to anything other than earlier Windows versions.
Interestingly, the last time I checked a few weeks ago, you can still order a copy of Snow Leopard from Apple. This is for them odd because they typically drop support after a 1+ OS iteration. The link is currently as follows:
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
I'm sure you've heard that a lot of people are now calling Snow Leopard Apple's "Windows XP." Some people just don't like the newer OSes, and apparently the fact that they're still offering it should tell you something.
Just my opinions and nothing more - please don't beat me up!