Minimum? You don’t really worry about how little one puts in, although it would likely be ineffective.
2% and below Salicylic acid is fine. Greater than 2% makes it an OTC Drug under the Acne OTC Monograph.
Lactic Acid doesn’t have an OTC limit but from experience I would follow the 10% AHA limit and the >3.5 pH recommendation. I know of a line that ignored this limit and faced a threatened lawsuit. In the lawsuit they cited the CIR limits, established the credibility of the CIR, and then alleged that the line ignored a credible guideline.