I wouldn't spend any more than 3 hours in QF-F. Enough for a nice, slow paced meal, maybe a shower. I don't think any lounge is worth staying > 3 hours (apart from those with bedrooms eg QR Al Safwa, where you could sleep off a long lay over, or the QR J Garden lounge with a gym where I could go for a run).
If I'm going to be sedentary for a 10+ hour flight, personally the last thing I would want is to sit in a lounge for 7 hours. I'd rather be roaming around the terminal or whatever to keep active.
But that's just me. I don't expect many to agree with this however!!