1. How long have you been doing these exercises? 
-I've been doing them for 1.5 hours
2. Since when have you been painting the picture?
-I've been painting the picture since autumn
3. How long has she been waiting for her lunch?
-She has been waiting for her lunch for 0.5 hour
4. Since when have they been growing these plants?
-They've been growing these plants since May
5. How long have you been staying at this hotel?
-I've been staying at this hotel for 3 days.