Indefinitely. So long as the secrecy is maintained, the life of a trade secret may last forever. In contrast, most patents are valid for 20 years.
A trade secret, by law, is defined as information necessary to make a product. The information must be as novel as patented information. The novel information is also required to be the key component to how you draw your profits. However, while a trade secret possesses the same characteristics of novelty as a patent, a trade secret may be preferable to a patent simply because it lasts so much longer. As of 1999, all patents are made public upon approval by the US government. This means that your entire patent is published online for anyone to download and view. In addition, most patents only last for 20 years. Once that 20 years runs out, then the information is in the public domain and anyone can use it to produce the exact same product.
Trade secrets are more advantageous for small to mid-sized businesses who may require more than 20 years to establish themselves. Trade secret law also provides more benefits than patent law for family-owned businesses who do not intend to hire large numbers of employees, since the threat of a trade secret being released in these circumstances is usually low. Finally, if your product is extremely successful in a highly saturated market, trade secret law will generally provide you more benefits, as it will ensure even more time for product success and development.
On the other hand, there are also instances in which patents may be preferable. Patents are better for products that can be reverse engineered, such as a new paint formula or a commercial recipe. Patents protect owners from having others reverse engineer (and then profit from) the product. Patent and trade secret law is very complex. If you have any questions or concerns about which category your particular invention falls under, consult with an intellectual property lawyer.