How many minutes before each prayer would be a suitable warning time?

Can you tell me how many minutes before each prayer ends would be suitable to warn a user of a prayer application? I would like to do this for these salats: Fajr Dhuhr Asr Magrib Isha For some of the prayers, I think they end at the following times: Fajr: Up to sunrise. Margrib: Until Isha begins. Isha: Until midnight If these assumptions are not correct then please let me know.