Permethrin's half-life is well documented and it is affected by environmental conditions. The reason it can last "up to 30 days" in reptile enclosures is that they're kept out of direct sunlight and while they're humid, they're not swimming in water. In a high-humidity enclosure like you'd have with a BRB where the substrate has more moisture or you're more likely to have condensation on the interior walls, the permethrin will break down faster.