Depends entirely on what your requirements are. We have a 2006 1,1 MacBook here that has been updated to 10.6. It had been running on 1.5GB since we got it (mismatched RAM because I had a spare 1GB stick to put in it, and they came with 2x512mb) and last year I upgraded it to 2GB and put in an Intel SSD. The optical drive has had to be replaced, and I think the replacement is dying now too, despite neither of them seeing much use. It's had one battery replacement so far, and is probably due for another within a year.
When doing anything more than simple web browsing (no flash) or listening to music, the fans spin up loud and the machine runs hot. It chokes on anything but the lowest quality web videos. But it's good enough for basic tasks, which is all it was ever really used for.
We have a late 2006 MacBook 2,1 running Lion (10.7) with 2x2GB RAM (only uses 3GB) and the original HDD. This machine feels a lot slower than the other MacBook due to the lack of an SSD, but it runs noticeably cooler. With this one, the battery is only now looking like it's going to need replaced, as we only get an hour or so of basic use (browsing & email) before it needs plugged in.
The case on both the MacBooks has cracked, but this was a known issue with the design, although Apple refused to replace them when I contacted them about it. (they didn't acknowledge it was a problem)
The screen on both of them is starting to look fairly dim these days as well, as they're CCFL backlit LCDs rather than the newer LED models. Both need to be run at full brightness to be readable in most cases.
At this point, I think if we ran into any hardware failures (HDD or battery are most likely) we would probably look into replacing the machines, though the price of Apple hardware has gone up considerably since these machines were purchased. The cheapest 13" model you can buy today costs roughly 50% more than those MacBooks did. (11" would be too small) I think one would probably get replaced with an iPad.
As the new MacBooks all have the rigid Aluminium unibody design (far more durable) run far cooler, and have a battery that has 3x the life expectancy (1000 cycles vs 300) I would expect to get at least six years out of it, just like we've managed to with these MacBooks. The biggest problem I foresee is that the battery is not replaceable. When the battery goes, that's it. If you want to be running the latest OS, I'd say you're sure to have at least four years before you have to upgrade.
As I said though, it depends on your requirements. Those MacBooks are fine for my family members, but as someone with high-end demands (editing RAW photos and upgrading cameras frequently for example) neither of those machines would be suitable for me, and I was wanting to upgrade my MacBook Pros (previously Powerbooks) on a yearly basis. Normally I would be able to hold out for two years, but when they made the switch to Intel, I had to break that cycle, and as soon as they released quad-core models last year, I knew that I would have to upgrade as that was a significant jump in performance, even though it would only have been a year since my last upgrade. If I had done that last year, I would be doing the same again to upgrade to a Retina MacBook Pro now.
Eventually, I decided that it was costing me too much to keep upgrading them on a yearly, or biyearly basis, so I made the decision to build myself a desktop PC and buy an iPad for portability to have the best of both worlds, rather than compromise on both with one machine. Turns out that was by far the best decision I could have made. I built it in February last year for 1/3 the cost of the base spec Mac Pro, and it still outperforms any Mac you can buy today. When it starts feeling slow and I want to upgrade (which will be my decision, as Windows supports hardware far longer than OSX) all I need to do is swap out the motherboard and CPU at a fraction of the cost of selling an old Mac and buying a new one.
