1. Since when has he been working at this problem?
2. How long have they been studying the contract?
3. How long have they been discussing this matter?
4. Since when have they been dancing?
5. How long has she been speaking on the phone?
6. How long have they been watching TV?
7. Since when has he been reading?
8. How long have they been doing this work?
9. Since when you have been playing computer games?
10. How long has my boss been writing a report?