Time entry cannot be restricted to specific increments. It can only be limited to hours or percentage. To answer your second query, you can set up a Validation rule for the Timesheet which limits time entry beyond the specified hours for a day. For example you set the maximum number of hours a user can enter in a day is 2, then the user cannot enter more than 2 hours for a day, irrespective of the number of Projects/Tasks. But, there is no option to restrict for a Time entry field.