Workflow task complition condision

https://sharepoint.stackexchange.com/questions/184726/task-completion-condition

You can have one service account which will act on behalf of someone. and add this account into your approval workflow. so when you will add item your workflow will create one task for person. and you can have one more workflow on this task list when will start on item create for particular service account user only and you can put delay their before updating task status.