There is actually a far more effective assay using P. lunula that takes approximately 4 hours to generate a response. The is called Qwiklite and is provided by a company in California called Assure Controls. I've used to assess to measure groundwater quality in areas where there is hydraulic fracturing, and I've been very pleased with how it works.
It depends which test you are using......are you using the chronic Microtox method or the usual acute toxicity method and just extending it. Microtox is a very crude measure of toxicity generally, and I wouldn't recommend using it alone to determine toxicity. Maybe try the MARA test (look it up on google) which uses 11 species of bacteria/ yeast, or preferably do some higher organism tests.