This is not a CRM issue as much as it is an issue with IIS7. I would point you to IIS7 settings. For performance reasons, user tokens are cached by IIS and updated at 15 minute intervals, in the CRM web site I think it is 20 Minutes.
However, think about this... If you were able to disable the token caching all together then your users would need to log in every time they requested a new record from CRM because the session would not have a token to allow a session to receive
the Data. So this seems a bit unreasonable.
But none the less here is a link on how to do it.
http://support.microsoft.com/kb/152526
KB article listed here details how to change the Default Interval for User Tokens in IIS. However.... I would tread carefully down this path as the CRM web site is set up to meet the applicaion requirements, so changing these setting may
have some unintended consiquences for your users.
Sincerely,
Pierre Hulsebus - EHTC - @infuseme