How to start Escalation timer when task is accepted by a user

You can achieve this via polling and task reports. You can check the task reports, say every 5 minutes, to see if a task has been accepted. For those tasks, you can calculate the accepted time by comparing assigned time and lag time. Then you can compare the accepted time with the time you want to launch your escalation, and trigger that escalation accordingly.

i am not able to implement this functionality in Escalation tab of User input task

You won't be able to leverage the escalation tab of the user input task for this approach. This workflow will need to happen somewhere else, either in a separate process model or in the same process model your task is in. For example, if you wanted to send an email to someone, you can use the send email node in conjuction with a timer outside of the user input task.

thanks Josh

thanks Josh