Various factors such as our load, the makeup of the email addresses uploaded, global network activity, and so on can affect how long a floss session will take to finish, but cleaning will always finish.
Our Deep Clean engine may take extra time as it does additional pass-thru and tests on harder-to-verify email addresses and domains.
Sometimes near the end, a session may look stuck at 99% as well. This is our anti-greylisting technology at work to ensure we can verify additional email addresses, as well as our Deep Clean engine double-checking harder-to-verify domains to ensure more accurate results.
If you feel a floss session is really stuck please contact us so we can investigate.