Photogravures will last as long as the paper they're printed on lasts without fading. They are etchings using fine printer's ink pressed into the damp paper base. Like any other medium, though, the more optimal the storage, the more permanent the image. So far, etchings have lasted for hundreds of years...etchings by Rembrandt date from about 1630. The "photo" gravure simply substitutes a photosensitive plate for a hand engraved one. The plate is then etched, if copper by acid, if polymer by water, inked and pulled through an intaglio press onto the damp paper, but the outcome is the same.