As you can imagine there is no way to answer the question regarding how long any one specific porcelain veneer will last. We can however provide you with an answer explaining, in generalities, the types of problems porcelain veneers can experience.
The life span of a porcelain veneer can be cut short because of some type of structural failure. Porcelain veneers can chip or break. The bond of the cement holding a veneer in place can weaken allowing the veneer to dislodge.
When it comes to repair porcelain veneers are a very unforgiving type of dental restoration. In the case where a veneer has come off and it is still in one piece it is possible that a dentist can reattach it successfully. With any other type of major structural failure it is most likely that the porcelain veneer will need to be remade. There is no way (in the sense that a porcelain veneer can be made as good as new again) for a dentist to glue the pieces of a veneer back together nor can they mend a broken porcelain veneer by way of patching it with more porcelain.
For the most part the only type of repair that a dentist will have to offer is to place dental composite (dental bonding) on the tooth in that region where the piece of the porcelain veneer has broken off. When this type of repair is possible it can in some circumstances provide a semi-permanent patch but it can't be expected to provide a long-term solution.
The reason for this is that tooth bonding will usually stain over time. This means that the patchwork will likely become fairly noticeable as it ages. Even initially however, since porcelain and tooth bonding each have different degrees of translucency a dental composite patch of any size, at least on close inspection, can't really be expected to provide an outstanding cosmetic result.
Porcelain veneers that do not experience a structural failure might still need to be replaced because their cosmetic appearance is no longer pleasing. Porcelain provides a very stain resistant surface for a tooth but the cement layer that lies exposed at the edges of a veneer can acquire staining as time passes by. As a deterrent to this problem a dentist will usually try to design the shape of a porcelain veneer so its edges are not readily visible. But if this thin layer of cement does become stained and this staining is visible then the veneer may need to be replaced.
Another cosmetic dilemma that can occur with teeth that have porcelain veneers is gum recession. Gum recession can take place for a variety of reasons, not the least of which is inadequate dental plaque removal from around a tooth. If a person's gums do recede then that portion of the tooth that has become visible can spoil the tooth's overall cosmetic appearance. When this type of situation has presented itself the solution is to make a new porcelain veneer. One that covers over this newly revealed tooth surface.