how to put an effect on armor for a while

...If you want to put some sort of special effect on armor, your best bet would probably be a custom enchantment. Make a custom enchantment, and have its effects execute on playertick if the player has armor with said enchantment. If you only want the enchantment to last for awhile, one of those effects could be that the armor has a very small chance of being disenchanted each tick. (If you wanted to make this a bit more complex, you could add a set delay before this starts taking effect, and another delay after tha rmor has decided to become disenchanted before it actually does. This way, it wouldn't immediately wear off.)

Do you mean for a while that an effect lasts a few minutes then expires?

In that case, check when player activates it, give a potion effect that lasts for the time you want and then a procedure to prevent the player from getting that effect during the effects duration. 

If that is what you mean.

Image is not showing, you can add a permenant varible after the effect, then check that the buff wont activate again on those entities.