Kristi,
If a user has the ability to create and assign themselves a task, they will also have the ability to change their allocation via the Gantt Chart. They just need to check the schedule out before making any changes to the Tasks. Once it's checked out, they simply need to double-click their name in the chart to make the adjustment to the % allocated.
Bear in mind that depending on the Task Type settings (Fixed Units vs. Fixed Duration, Effort Driven, etc), modifying a user's allocation % on a task may affect the duration of the task.
Another way a user can adjust the planned work (hours) within a scheduled time period (duration), the task can be edited to explicitly set the number of hours within the scheduled duration. Again, the Task Type settings may affect the behavior of these attributes.
Hope that helps!