Instant film contains multiple layers. One of them is a regular film negative, which is why the photo is sensitive to light until it is fully developed. The development time of the color Impossible picture is about 30 minutes. The reason for this rather long development time is mainly caused by the so-called opacification layer. It is another, blue, layer within the film, which protects the lower (light-sensitive) layers from light. Think of the opacification layer, or opacifier, as a chemical curtain that needs to be drawn over the film to protect the image you’ve created.
The opacifier isn’t perfect protection: Therefore we recommend using the "Impossible Frog Tongue", a device that can be installed in most Polaroid 600 and SX-70 cameras which helps to further protect your pictures:
http://www.shop.polawalk.com/product/frog-tongue-boxAvoid exposing and developing in direct sunlight; this will cause overexposure and reddening of the final result. The Impossible instant-photos are no longer sensitive to light after a few minutes but it will take about 30 minutes to fully develop the Polaroid. It might even develop further after that point!
In short: Expose your Polaroid as little as possible to sunlight.