The time between when you make a donation and when the candle is actually lit in the church of your choice depends on many factors, such as the quality of your Internet connection, the distance to the church and the number of candles that are already lit in the lamp. The average time to power up is about 5-10 seconds and will rarely exceed 30 seconds. In all these cases the app will notify you when your candle is lit so you can check it.
While your candle is waiting to be lit (because you have already made your donation but it is still being processed), you will see in the app that your flash is listed as "Pending" and the main screen of the app will look different.