how to fix time duration in online examination system by admin

You're going to need a database table called something like 'exams'. In the below, duration is in seconds

exam_id description duration
1 Easy Exam 1800
2 Hard Exam 900

When the user starts an exam, retrieve this from the table. Note the time they started in the session + you'd probably want to store in script as well so you can offer some client-side functionality

Warn user on client when running out of time

Reject posts to server when user has exceeded exam duration