when will your skin get 'used' to products?

[fashionlives]

i wouldn't want skin care products to get 'use' to my skin so the good effects won't take place any more if it is immune to the ingredients... also i don't want my skin to be dependent on the ingredients either.... is this possible

and

does anyone knoe a 'safe' amount of time to use a product before switching products (even for a while ) to prevent such predicament from occuring?