A reality check is a tool to remind you how long you have been gambling. It does not control session length. The reality check allows you to display an on-screen message which shall appear at regular intervals, according to the time of your choosing.
When the notification appears, you can either continue playing or stop your gaming session.
To set a reality check, when logged in:
Go to your account
Navigate to 'My Account' from the menu option
Select 'Gambling Controls'
Then select
'Reality Check'
Here you can set a time in minutes of when you would like your reality check to appear.