How can we make registered users appeared at who's online block, after they left our site for a short while?

i mean

i need a delay for disapperance of them from that time they leave my site..

The block only displays all the current users for which a session exists. There's no way to tell if they are still there or if they have closed their browser. The session will expire after a while and then their entry will disappear from the block.

The Visitor Stats plugin (now called GUS plugin) has an option to list all the users who logged in today. Maybe that is closer to what you want?

bye, Dirk