There is limited scientific research specifically focused on the use of Castor oil for hair regrowth, so it's challenging to provide a precise timeline. Some people may notice visible improvements in hair growth and thickness within a few weeks, while others may require more time to see noticeable changes. Consistency and patience are key when using Castor oil for hair regrowth. It's advisable to apply castor oil to the scalp and massage it in gently, ensuring that the oil reaches the hair follicles. Leave it on for a few hours or overnight before washing it out.