The session-time defines the time till the customer is logged out automatically by the eshop when he is doing nothing.
This value is in standard 30minutes or something like that. If the customer is doing nothing for this time he will be kicked out. Then the message "Session no longer valid" occurs.
This is a parameter which can be set in web.xml as i described before.
If you set this value to "2880" the session is valid for 48 hours.
If you do that, its not needed to change the message because the message never will be displayed again.
I think this would depend on the number of sessions that you have in yoru system. If you set this to 48 hours then the session clean up will only be done at this point. If you had a large number of idel session this could result in performance issue. But this would depend on how many users would leve the browser open for this period of time or close the browser without loggin off.
If your customers are not longer than 48 hours logged in to the eshop, then this is working fine.
Maybe you have to increase the system parameter CPIC_MAX_CONV too.
When the session-time is longer, the RFC-connections to your Backend are longer needed and not closed. If you have many customers on your eshop, then the standard value of max connections to your backend is not enough (Standard is 100 connections).
You have to set that parameter on your ERP/CRM, Netweaver-Server, TREX.
You can find more informations about that parameters at the note 1147336.