This setting allows to change the behavior of the "Online" counter field on "My Projects" page.
The default setting for this counter is 20 minutes, which means that the visitor is still considered as "Online" although the last activity was detected within the last 20 minutes.
Since it may take a while for people to read or view your online content, which could also be in incremental manner, the 20 minute timeframe is determined to be reliable average that applies from reach in reading content to multimedia rich websites.
If your website is extremely busy and you have hundreds of visitors, you can decrease this value to get a better idea of highly active visitors.
For best results, you can approximate the time needed for an average person to read a page on your website, and set the timeframe of this setting to your best guess.