There are several ways to get there.
If you goto:
Control Panel Â» Group Management Â» Forum Permissions for Users group
Edit post time limit
Enter the time in minutes for each desired forum.
If you go another route then you need to use the switch button or you are editing the default group template not the actual group.
If indeed the settings are not saved then visit:
Control Panel Â» Permission Checks
And make sure all the lines show okay.