This is enabled in the User Profiles program.
Open User Profiles
Select the User you would like to amend
Go to the Task Diary tab
There is a section labelled Pop Ups and there are two options:
Minutes before task time - this sets how long before the task is due that the reminder will appear
Scan tasks every minutes - how often the User's task list will be checked for due tasks
Press Update, enter the desired settings, then press Save
The User will pick up the changes on their next login
If you have any issues setting this up or do not have access to User Profiles then please raise a new case online and reference the title of this article.