I wouldn't be focusing on time spent.
Time is not in a direct correlation with effort and while effort should strongly correlation to marks - obviously other things effect that as well - perhaps deeper understanding of subject or certain areas or even teaching.
Lack of effort would be bothering me - and while my DC are still at primary I'd probably do what some of their GP did and sit down and talk to them about how it's easier with good exams results and how effort now pays off and see if there are reasons I could help with why effort isn't being made. Does she need help managing her workload better - estimating times needed?
By this age both DH and I were managing our work load and time - parent asked questions but didn't dictate when or how long we took to do things.
Having said that I don't get the out rage at the two hours a night - mon to thur I regularly did 6 to 8 /8.30 and time in lunch or weekends wasn't unheard of in fact happen more often than not.
I chose to do that - though later diagnosed with dyslexia so probably was working harder than many other students - but a rural location meant limited social and outside hobby distractions for the most part.
I'm not sure I want the same of my DC - I think I'd want more outside pursuits and interest though DH thinks he was doing about 2 hours most nights at this age - he got in before 4 did his food then sat down till his parents made it home around 6.30 - 7 then often went out later round his mates or to his group activity.
We were both at fully comprehensive standard state secondary's in very different parts of the country - though we did better than most who went there.
DH family did incentives - bring home so many A and you get x- I really don't think they needed to but they did - mine praise effort throughout my time living with them and got me to focus on what I wanted in future and what I needed to get to get there. Basically they were getting us to motivate ourselves - have you tried stuff like this rather than insisting on a set amount of time?
Or could you try insisting that HW is done before she gets to do other stuff at weekend - switch the internet off all weekend, lock the doors - I'm half joking there but mine at younger rage have to do things before TV, or minecraft are put on - or raise it fri, sat and see if you can get her to sit down for few hours then she has Sunday to polish it or if not at least to estimate better the time needed to do task ?
Or if you've tried that - would her doing badly this year act as a wake up call for next year or term?