How long is lentil soup good for in the fridge?

It's good for at least a week if you use tomatoes. The acid will help with the keeping.

I have a five day rule for anything left over.

When you heat the soup, bring to a point just short of boiling, for an extra measure of caution. If it has gone 'off' you can tell -- by sight and by smell, or a drop on the tongue. If you have meat or meat broth in the soup, there is less margin of error than for a meat-free soup.

I second what Mrs. Larkin said.

I think it should be fine. But if you don't finish it today, I would freeze it in single-serving portions, either in freezer-safe containers or small freezer bags.