This depends on a LOT of variables.
SD Cards generally will have about 100,000 program and erase cycles, with some of the better cards maybe having a million or more. How often you use the card, how much data you transfer each time, etc. will 'eat up' these cycles. It's kind of like shutter life in a camera. When Canon tells you that X model camera can handle 150,000 shutter cycles, it doesn't mean that the 150,000th is guaraunteed to fail, it just means that you should be able to expect at least that much performance out of it.
Generally, an SD card will be outgrown before it's worn out. I have some SD cards that are a few years old but they are 512MB, 1GB, etc. Not particularly useful in a 15MP DSLR shooting RAW. So they work, but they've been replaced by (currently) Sandisk 8GB cards.
If you are worried, replace them. SD memory is cheap. Cheaper brands will wear out much faster, generally speaking. (Not always though). I've always heard the SD cards are most reliable after about 2 weeks (if it's going to fail early, it'll fail in that windows) and before about 2 years. That's probably a good average, although again, it depends on SO many variables. I have an old old 64MB (got GB!) SD card sitting in a drawer, and just for grins I checked it, it works. But I don't even recall WHAT I used it for, much less how often I use it. It will probably outlast my Sandisk 8GB cards that I use all the time, when it sits in a drawer.
Again, if it concerns you, replace them, you don't want to lose your pictures! I tend to lean towards the name brands of flash memory too, they tend to be more reliable. If a couple years is two, then some would say you are venturing into unreliable territory. FWIW I tend to use several smaller cards over a couple larger cards, in my mind, should I have a total failure of a card before I copy the images to my PC and subsequent backup drive, I'll lose less images.
I don't know if any of that was helpful, but unfortunately, to my knowledge this isn't a finite issue with an easy answer that works for everyone. You may have heard about optical media degrading, well, that's due to sunlight, humidity, etc. They physically degrade over time. This doesn't necessarily happen in properly stored SD cards (aka don't store it in your back window of your car in pheonix in July), so their life is more dependent on how and how often they are used.
Also note that most of your name brand cards will have 'load leveling' chips in them that will make sure that everything is read and written 'evenly', this leads to more reliability because there isn't a 'hot spot' where one particular area is getting written on over and over again. But, in the end, you really AREN'T likely to use up those cycles. Flash memory IS considered more reliable, but we've all had cards fail. I've only lost two cards in recent memory. One still technically works, but the plastic casing came apart. It broke taking it out of the camera, I was able to get the data off, works fine but I of course won't be using it any longer. I have another one that I used to use in an mp3 player that has failed completely. Compare that to a couple dozen working-fine SD cards. But that's me, I'm not you!
I think it came with my very first Canon Powershot, and that's been QUITE some time ago.
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