You will feel the cast forming quite quickly - within the first 10 minutes of applying the gel. This is why it is important to make sure your hair is sitting in its natural position before the cast begins to form. Once it has formed you run the risk of breaking the cast too early if you move your hair. You will need to leave the cast until your hair is fully dry - however long this usually takes by air drying or diffusing. If you aren’t sure try carefully sliding a hand to the underneath of your hair near the nape of your neck. You might be able to feel a root area which has not been covered in gel to check whether your hair is fully dry.