Not sure, butif you don't want to waste it, freeze the leftover lentils immediately after completely draining any cooking water out of it.

I see no need to freeze them if you're going to use them in a week. I would drain them and refrigerate them in an air-tight container so they won't pick up any unwanted odors. Another way I sometimes save them is to store them in a vinaigrette. Then they're ready to embellish a salad in a minute.

3-5 days is the usual practice for cooked lentils. A week is pushing it. If I knew I wouldn't use them for a week, I would freeze them.
http://foodstorage.pbworks.com/w/page/55471245/Lentils

Stilltasty.com says 5-7 days in the refrigerator http://www.stilltasty.com/fooditems/index/17548. At a week, I'd be should be sure to cook the soup well.