Seems impossible to me actually have a 100 GB on device like phone and try to back up it via WiFi or 4G, just saying it is not usual and not a lot devices are supporting such big SDXC. Anyway in case you will using your WiFi modul please see the max. transport rate on your router (could be about 76Mbps or 1600Mbps) - from this you can guess the time for backup. In case you are planning to use backup from avast, please check you free space on Google drive.