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.
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
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.
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http://foodstorage.pbworks.com/w/page/55471245/Lentils