if its done properly it will last forever, clean, dry glass & mirror bracket is the secret, dry/warm with a hair dryer, use meths to clean, NOT white spirit or glass polish as these all have some form of oil in them.Make sure ALL the old sticker is removed from the screen and mirror, a bit of wet & dry paper on the mirror bkt can help.
Also certain stickers have one side green, this goes 'green to screen'. If its not done right it will be off again in a matter of days like i said done properly it will be bombproof .