However there is no specific time because snapchat has not declared any specific guidelines regarding this. But as an assumption it takes up to 30 days for them to respond to a data download request, the actual time frame can be depending on various factors such as the size of the data being requested, the volume of request they are receiving at any given time and the speed of your internet connection etc.

For more read - https://allinsider.net/how-to-download-snapchat-data/