Lots of bulldust in this thread, and it looks like most of it is from people who don't ride with specs.

I'm as blind as a bat without glasses, so they are always on my face. I have to live with the rotten things which is a bloody sight different to those who buy expensive sunglasses because they imagine they help their cycling/life/fashion/whatever.
I don't bother about them very much except to try to keep the lenses clean - not always possible with sweat or oil or any of the other zillion things we seem to cop in our faces. Anyone who genuinely lives with glasses will tell you that sometimes, nothing will get them clean and, when out on a ride, sweat is the worst of the lot.
My glasses frames last for years, far longer than the optometrists would like to see them last for the simple reason that I can not afford to keep changing glasses every time time the fashion industry dictates something different. I typically get five years or more out of a set of lenses and frames, and that is with riding almost daily and from a person who sweats like blazes, even in cold weather. The ONLY time I've seen a deterioration in frames was from metal fatigue, never from sweat related corrosion.

So, will sweat affect your glasses?
Probably, but it's not a problem, especially if you're someone who likes new glasses every twelve months or so. Just ignore them and replace them when they don't give you the clear vision you need.