How quickly do cheap domed stickers start to yellow?

There is no fixed interval, because the rate depends on ultraviolet dose, ambient temperature and the specific formulation. What is predictable is the direction. An aromatic polyurethane or epoxy dome placed in direct sun will shift towards amber and will not come back, whereas an aliphatic polyurethane dome has no aromatic ring available to form the chromophores responsible. Where a service life must be evidenced rather than estimated, accelerated weathering data to ISO 4892-2 is the appropriate request.