Check the CF Admin. The values set there can trump per-application settings. So if the CF Admin states that the maximum size a session can be is 10 minutes, your app can tell CF to use more, but CF will only allow it to be as large as what's set in the Admin.
SERVER SETTINGS > MEMORY VARIABLES > MAXIMUM TIMEOUTS > SESSION VARIABLES
Also, FYI, you do not need to pound off the createTimespan() Built-in Function when you set it, ie, this is perfectly fine:
<cfset this.sessionTimeout = createTimespan( 0,0,45,0 ) />