The interval between them for Fajr is the time it takes to recite twenty verses, and this is not a strict measure. It should be done in a way that allows people to gather, considering the preferred time for Fajr, which is to delay until there is enough light to recite forty verses or more. Then, the prayer can be repeated if it is found that the ablution was invalid. Refer to: Fath Bab al-Inayah 1: 202.