CSP/ZEN page session timeout while extracting huge amount of data

I don't know what you're extending your class from, but try calling ##super() before you set the timeout.

EDIT: Hold on, try using %response.Timeout instead. Maybe your problem is not the session itself, but the response timeout instead.

/// Can be set in the OnPreHTTP() method of a page in which case this changes the amount of time the
/// CSP gateway will wait for a response from the server in seconds before it reports the 'Server is not
/// responding' error message. This is useful if you know that this page is doing an expensive SQL
/// query that will take a couple of minutes and you want to set the server response timeout on
/// the CSP gateway to a minute and yet wait three minutes for this page to respond. It will just
/// change the server response timeout for this page only. If not set the the CSP gateway uses its
/// default timeout value specified in the CSP gateway configuration.
Property Timeout As %Integer;