I agree with the others. Three years easy; five years really depends on what you mean by "not a heavy user." I'm typing this on a 5-year old iMac running 10.6 and the 2008 version of MS Office, and it's certainly more than fine for that sort of usage. My wife's 15" MBP is still going strong after four years, but she does not do any video editing on it. Neither, of course, is as snappy as the MBA 11 i7 I just picked up, but I would expect both to last for at least another year or two. Same for my old 17" MBP from early 2008, which I just gave to my niece. The build quality of Apple computers is first rate (and should be given their premium prices), so that's not been an issue for me. I'm not sure I'm going to install 10.7 on either of the older machines I still have, however. I suspect that if you go to the G4/5 board, plenty of folks there would tell you that their computers still run fine, albeit with software limitations. My brother-in-law still uses my old 12" MBP, which must be over 6 years old by now.
As far as dropping the computer, I suspect any laptop (Apple or otherwise) will break if you drop it just right, unless you get one of those milspec machines. The biggest concern I had with the MBA is that it is so slick and small, so I put a Wrapsol skin on it to give it some grip, and I use a case (BookBook case) when traveling with it. I've used Thinkpads at work for years, and those computers can take real abuse. I think that their composite/plastic cases actually help, because they have a a little give. I've traveled a ton with them, and just throw them into my briefcase without any cover or case, and have never had a problem. They refresh our computers every three years. They do show wear, and I'm not sure how the newer Lenovo versions will do, but I'd buy a Thinkpad laptop without hesitation if I had to get a Windows PC.
Personally, I think that the computer industry tries to make computers obsolete quicker by writing more and more demanding software that really isn't necessary for the vast majority of users, including me. But my needs are pretty limited.
Good luck. FWIW, I love my MBA 11, which is maxed out, and I hope/expect it to last a good four years.