How long does graffiti and street art last if not deliberately removed, buffed with a fresh coat of paint? Minutes, hours, days, weeks, months, years, decades, even centuries. A medieval peasant cutting into the painted plaster on the church walls (for more about Medieval Graffiti). An ancient Roman scratching images of gladiators into the stones of the Colosseum.
Short answer: It depends on the medium used and the location.
Spray paint fades over time, especially in the full sun, but it does last for years, decades even. However, the same graffiti writer will paint over their own pieces to keep the paint fresh. Often it is only when a wall becomes inaccessible will they cease updating their piece.
Unlike graffiti, street art is not updated or replaced by the same artist. So the permanence of the media that a street artist uses. Paper and paste are surprisingly durable but will eventually deteriorate in Melbourne’s much-discussed weather. In the harsh and unforgiving outdoor conditions, there are casualties. Parts rust away and fall of pieces of rubbish nailed together by Junky Projects, making them meaningless.
Other media, like stone-carving, concrete casting, like Will Coles or Sandor Matos, or ceramics, lasts longer. You might be surprised at the number of unauthorised mosaics because you would think that there was almost none. Ceramics have been used as a medium for street art for decades, from the tiles mosaics of Space Invader to the work of Far4washere, a Melbourne based mixed media artist. The durability of ceramics to weathering on the street means that they have been used for authorised street mosaics (see my post about mosaics in public art in Melbourne).
The idea that graffiti was a fad contributed to a sense that it was ephemeral. The fact is that graffiti and street art are often on walls that nobody cares about; even legal projects used to bandage over an aesthetic sore spot. The building may be abandoned or scheduled for demolition. For this reason, development and other building work (a plumber putting a pipe through a Banksy) make graffiti and street art ephemeral. In Melbourne, Blender Lane, Centre Place and Lovelands are three street art/graffiti locations that have been significantly affected by developments.