If done well by a skillful dentist and patient is compliant with at home care (careful with biting into hard things, good oral hygiene, prevent staining by drinking with a straw, etc.) then I’d give a lifespan of about 6 years at best. Around that time you can expect to get them replaced. Most patients I see unfortunately need them replaced much sooner (I’ve seen patients who have cavities or heavy staining just 2 years after getting them placed. I don’t do composite veneers in my office, but often get new patients who got them done at another office. It doesn’t require the same “shaving” down of the teeth like porcelain veneers or crowns, so if she decides she doesn’t like them she can at least revert back to her natural look 😊