It depends on the product. If you’re testing a granola bar, a few bites may be enough to write a review. If you’re testing a pair of athletic shoes, the manufacturer may want you to try them out for a few weeks before you give your opinion. In the case of a granola bar, the product test may consist of a short survey where you answer a few multiple-choice questions or write one or two paragraphs. But let's say you sign up to be an Adidas product tester; in this case, you might be asked to evaluate a pair of running shoes, which could require keeping a detailed, weeks-long record of how often you ran, how far you ran and the conditions (hot/cold, rainy/dry, etc.). In other words, the expectations and amount of work required varies dramatically. One thing to keep in mind is that once you commit to testing a product, you need to complete the product review. Otherwise, you'll be unlikely to receive future shipments from that particular product testing website.