Yes, you can leave hair dye in for longer than 25 minutes. The amount of time you should leave it in depends on the type of hair dye, how dark you want your hair to be, and your own personal preference. If you are using a permanent hair dye, then it is generally safe to leave it in for up to an hour. If you are using a semi-permanent or temporary hair dye, then you may want to leave it in for only 20-30 minutes. Ultimately, it is best to follow the instructions on the hair dye package and test a small section of your hair before applying the dye to your entire head.