Hi @Muthuraman Ramasamy , thank you for posting in the Q&A forum.
After enabling and setting Idle Session timeout via Central Administration / using PowerShell / in Web.config file, you can write your custom function in a custom master page in the SharePoint root site collection. A sample can be found here: Sharepoint timeout.
Once you finish building the custom mater page in root site collection, you can apply it to all site collections in this web application using PowerShell, referring to this post.
Another reference of JQuery Show Session Timeout message before session expires in asp.net.
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