Session timeout is website based. You define that in Web.Config file.
You cannot have website session timeout as 20 min utes and a control in the website as 30 minutes timeout!
I would suggest you to modify the session timeout of the website as 30 min, via Web.Config:
<configuration>
<system.web>
<sessionState mode="InProc" cookieless="true" timeout="30"/>
</sessionState>
</system.web>
</configuration>
To read more about it, look
here.[
^]