Timesheet is waiting for approval notification is sent out the moment timesheet is submitted. However there could be scenarios where there is a Time off on the timesheet waiting for approval, and there are different approvers for timeoff and timesheet.
 
In cases where the timeoff approver has not approved the time-off, then the timesheet cannot be approved and will be on hold till the time off for that period has been approved. The timesheet is waiting for approval notification will also be waiting for the timeoff to be approved as the timesheet will be queued for approval only after the time off is approved. 
 
Hence the Timesheet is waiting for approval email notification would also be sent only after the time-off is approved.