Sunshield, I've RTFT. There are a whole heap of interesting discussions, comments, insights and opinions in the 300 messages.
I think nearly every post you've written comes back to focussing on time spent on homework. Only in one or two posts have you written about the outcomes of your DD's work, or her work pattern.
I think that you are focussing on completely the wrong thing to measure. Time spent versus results achieved.
From the one-two 'useful' posts, it does sound as though your daughter is not achieving to the best of her ability, and is not trying to the best of her ability.
However, if you keep bashing her over the head with the clock-watching, I can't see that you're really going to achieve much, apart from hacking her off even more entirely.
What are you doing to support your DD rather than have a go at her? Do you encourage her to do her best for positive reasons (choice in the future) or for negative reasons (you'll amount to nothing if...)?
I appreciate that your own educational experience has affected you strongly, and also may have left you feeling as though you have insufficient knowledge to work with your DD, but I think you need to find a way to do just that - work with, encourage - or you will end up with a very switched-off child.
As for how that support and "working with" might work, I'm not sure. How about helping her to break things down into managable chunks? How about more interaction with you? - Praise from you about a good piece of work done/ a bit of a kick up the arse when it's not done as well as it she could as she's being lazy or rushing something? Helping her to understand that most of her peers are probably lying about doing nothing? Apologies if you're doing all of this and your post is simply to get feedback on what is a reasonable amount of time to spend on homework, but I get the impression that you feel that your role is just to police time, and not to be involved in educating your DD because "that is the school's role".
Anyway, good luck. Stroppy teenagers are stroppy teenagers and difficult to motivate unless they want to do something.