It depends on the condition of your lenses, if they haven't been scatched, generally speaking, a quality anti reflection coating should last at least 2 years. Make sure never wipe your lenses when they are dry, but rinse them first under warm water, then use the special cleaning cloth wipe it. If they are scratched within 2 years, you jusy need to replace it with new lenses.
If the anti-glare coating is good, it will last at least 2 years. Some better one will last even longer. I have worn a pair of glasses with anti-glare coating for 3 years, and now it is still in good condition.
If you pay enough care to it and don't make it scratched,it can last as long as many years.You should be especially careful when cleaning it.I have a pair of anti-glare glasses and they are in the third year now.