Assuming you are referring to the Combo Cleaner app for Mac:
According to the official website, a full scan of your system with Combo Cleaner can take up to 30 minutes. However, they also note that the duration of a scan can vary depending on the size and condition of your system.
If you have a lot of files and/or programs on your computer, it will naturally take longer for the app to scan through everything. Additionally, if your system is already running slowly or has other issues, this can also impact scanning time.
In general though, you can expect a full scan with Combo Cleaner to take around 30 minutes.