1. We have been in this store for 20 minutes, and we haven't bought anything yet.

Здесь используешь Present Perfect Continious.

2. What is he doing? - He is playing tennis. - How long has he been playing tennis? - 2 hours.

Здесь используешь Present Perfect Continious и Present Continious.

3. They have been traveling for two weeks and have called two times (or twice for example).

Здесь используешь Present Perfect Continious.

4. Where is Molly? - She is ill. - How long has she been ill? - She is been ill since Thursday.

Здесь используешь Present Perfect Continious.