It's not only when talking about maths, it's a question for every subject you learn in school: what's the right balance in terms of homework?
The one thing is certain, it's not easy to answer and it's definitely not one-size-fits-all type of answer.

When I was in school, I never really liked homework. I guess it was mostly because I didn't need it. I was able to catch all I needed to learn, all the new stuff, just by being in school (and paying most of the time an attention what's going on there during lessons). So I didn't feel I need it, therefore I didn't like to be given homework.
But, of course, I understood, the main purpose of giving homework to students is not to annoy them and steal their precious free time (sure it might look like sometimes, but that's not the case =), it should give them a reason to practice, therefore to get better into the topic and help them to understand and learn it.
You could disagree by saying "if I'd need to practice more, I'd just do it", well, maybe you, but not everyone has developed (yet) a good level of responsibility to work on their own, so they need to be pushed (so in terms of homework from school, it's just for everyone, teachers can't really check everyone's level of responsibility for their own active learning outside of school).

So here we are getting closer to the topic. I can't really influence how much homework do you get from your teacher, but I can tailor it to your needs when we have lessons together. Sometimes parents are asking me to give loads of homework to their children I'm tutoring. But too much is as useful as too little - it's simply not good.

I can hear complaints of those who just say "but I want to see the best grades, we want to get A*******" - well, one thing is what do you (or your parents or your teacher) want, another thing to consider is what are you actually able to achieve. Yes, I had students who did their best, worked really hard and yet didn't get the grades they wanted, some of them (not many, fortunately over all the years as a tutor I had mostly two or three students who simply couldn't achieve what they've set for themselves) even don't pass - and it might be not their fault, really.
I had even a colleague at the university when studying maths, who was really struggling with it. She was trying to pass exams from the first year so many times, even enrolled again to the first year after she used all her attempts and therefore she couldn't continue in her study program. I asked her "why"? Why do you keep torturing yourself with university maths, when it's probably not the best thing for you? She replied she simply likes it. Well, in that case, that's fine, good luck and let's see. But you have always choice here to choose to study something else. 
Some people excel in history, yet maths could be not their thing. Good thing is, you need just a certain level of maths, here in the UK some good grade from GCSE is all you should be bothered when you're life isn't going to include any economics, science, IT and other more maths-related pathways (unless you simply like it, as my colleague, and want to try again and again). And to get a sort-of-good-grade in GCSE - good news - I believe everyone can achieve. Well, you may have special education needs, then you may be limited, and then that's a bit different topic, of course.

And proper amount of homework is meant to help you to get there. That applies to every school level. So now we see what we can achieve and why homework is important, let's think about how much is actually good?

As I said, there isn't any one-word answer for everyone, but I'd simply say - too little is when you don't feel confident yet with the topic, too much is when you reached the point where you were confident and did everything excellently and efficiently and now you actually start doing mistakes or after first excitement of "finally properly getting it" you start to feel bored and not excited by doing any more of the topic.
The catch is, some (most? =) students feel bored from the very beginning. The important thing is, first you need to feel an excitement of understanding the topic, then you'll get bored when you don't move to any new theme and just keep repeating what you've done so far so well.

Too much homework could be also when you have simply too many subjects to focus on, or, of course, also other non-school activities and commitments to catch up on. Then you need to sort out priorities first, because, no matter who you are, everyone in this world has only 24 hours per day =) So you should choose wisely what to do with it. I know at a certain (younger) age you cannot fully decide yet on your own what to focus on because there are other people around you who link your path more or less. Then it's up to you to discuss your needs with them and find the best way for yourself and not to be overloaded with (maths) homework only when it's not what you decided you will need in your life most.

But to not to get me wrong, I do believe in homework, but it needs to be reasonable. That's why I mostly get to know my students first, then I'll find what's better for them. If to set any homework, then if it should be me telling them what I want them to do, or if I should only guide them and let them choose what they feel they need to practice before the next lesson.
And so far it seems it works well. If I see you need to be pushed, I'll do it, but I know, too much is as good as too little - and that means actually not really good =)