Do I have to buy new one every 2-3 months to swim fog free? Is there a way to protect the anti-fog feature for long times?
There are a few low tech methods:
Toothpaste (Smear a bit then rinse free, be cautious as some people are sensitive to toothpaste around the eyes)
Anti Fog spray
I've had good results with the aquasphere line, but eventually all glasses fog over. I usually just spit on the insides, smear around, then rinse them out and slap them back on. During practice, I generally just smear pool water over the inside with my thumbs. After a while, fogginess just doesn't bother you any more.
"Fogging" on the inside of the googles is caused by temperature differences between the inside and outside of the lens. This is the same reason your car window fogs up when you turn on the heat in the winter.
The simplest solution is to dunk your googles in the pool before you put them on. I've done this for over 8 years as a swimmer and I've never had issues with fog, no matter how old the goggles were.
I used to rub them when I was a kid but then my coach showed me the dunking technique and it changed my life forever... :)
My experience: put your goggles into the water, salivate by your tongue and put immediately on.
EDIT: I'd never use any anti-fog chemicals now. Why? When the protection is partially off and goggles start slowly fogging you will be forced to think about licking chemicals in case you would like to use my tip. So don't use any chemicals, just salivate it. I'd like to have something chemicals free on my eyes :)
A cheap and fast solution is using saliva.
I use quite frequently in all my glasses (swimming and spearfishing) since ever. It is quite effective.
Water the lens. Spit on the inside and rub it. Wash it.
For spearfishing there are also some film one might use on the lenses (they solved the fog problem in one of my masks) :
A more expensive solution, that I know of, is athletes that in every competition wear a new set of goggles.
Here is a good list of potential solutions. Even though it is for a scuba mask, it pertains to any goggles as well. (ex: burn glass with lighter, toothpaste, saliva, baby shampoo, glycerin soap). http://scuba-diving-explained.com/no-more-fog-in-my-scuba-mask-secrets-to-a-clear-mask
Instead of buying anti-fog goggles (which no swim goggle is truly anti-fog--they all say so, but it's one of the biggest jokes amongst swimmers), make sure you are buying swim goggles that are:
- Light for indoor use. Dark tinted goggles look cool, but aren't super functional for poorly lit pools. Especially when the anti-fog eventually wears off.
- Dark tinted for outdoor use. Seems obvious, but you'd be surprised how often swimmers show up to outdoor lap swim with clear-lens goggles. They end up squinting through their swim, unable to swim backstroke without closing their eyes (and swimming into the lane rope and other swimmers as a result).
The best thing you can do is to keep your fingers out from the inside of the lenses--wiping sweat, water, whatever--will peel away the film of anti-fog.