That depends on your region and the type of product. In Flanders, small packaging up to 30 litres must be notified from 50 litres or kg in total. In Brussels, you already need an environmental permit from 50 litres of highly flammable liquids, and in Wallonia the declaration for the most flammable category starts at 50 litres.
The federal rules of the Code on well-being at work also apply from 50 litres. All thresholds by region are in our guide to the storage of hazardous substances.