You should be able to configure this timeout under Url Filter > Quotas and Coaching > Session Length for Quotas.
If you set this value higher, users should not be warned as often.
Thank you for your answer. I have configured this option about 2-3 weeks ago and have set a value 60 minutes. But notification page appears regularly in different time interval - after 2 minutes start of session, 5 minutes, 30 munites and other. So that this option didn't help. May be this issue caused with other reason?
I recommend to file a ticket with Support, so that we have a chance to take a look into your configuration and the network environment, in order to analyze what is going on. I guess this will be the most appropriate way to have a quick solution, instead of discussing this within the community. We will of course post the results once we have foud the root cause.
To file a support case please go to http://mysupport.mcafee.com or call in the Tech Supports hotline.