As far as I know there isn't a standard length. Some guidelines for adult zebrafish toxicity studies say around 4 weeks, but in everyday lab work, I have done studies with 1 day, 2 weeks, and 5 weeks. For embryos, I usually see 5 days or less. The different lengths "model" different patterns of exposure, like a poisoning event, seasonal exposure (subchronic) or chronic sustained exposure.