I just bought some "chicken fillets" (cleavage enhancers) from anne summers and on the back of the box it says not to wear them for more that eight hours......
Its only really beacuse in case your skin gets a bit sensitive to the product, its just for their own protection i suppose, and normally applies to the enhances with a sticky side, although ive never had any probs wearing them for more than 8 hours. I put them in pop socks (tights) to stop them becoming sweaty in my bra.