IE timeouts on Digest-authenticated IIS web site

Windows 2003 Server SP2, IIS 6

Problem: Users must reenter username and password on Digest-secured site
every 10 minutes or so.

After experimenting with various IIS settings, we have been unable to manage
the duration of secure user logins. We're beginning to think that IE may
(also?) have a timeout. The question is: What settings must be performed to
lengthen the time between user credentials using IE and IIS 6?

Thanks,

TedMac