Most scans should take less than 10 minutes, although this can vary depending on the device and the number of apps/files. If your scan is taking significantly longer than this then please consider the following.
During a scan, if you minimize the app or lock your phone then the scan will continue, but will take significantly longer. This is because scanning is slower when running in the background.
For best performance, keep the app open and your phone unlocked when performing a manual scan.