I know this has been discussed before and I know it's a bit of a 'how long is a piece of string' type question, but does anyone actually make recommendations about how long pupils should be practising at particular levels?

Obviously, the primary concern is that the practice is about quality not quantity, but even though I make this point, pupils and parents are often keen to know a 'time'. I'm not talking about arguments like 'you need to practise 10 million hours to become a professional' etc. etc.

If we said for arguments sake that the practice is focussed and effective, what would be a good suggestion, say for beginners, Grades 1-3, 4-5 and 6-8?

Half an hour, and hour, one and a half to two hours.
Though I think many do well on maybe 45 mins at grade 4-5 and an hour at 6-8.

If they are looking a at time maybe you can qualify this by saying that there can be some flexibility in this.

My friend who is an adult beginner said that her teacher requires half an hour almost every day (as in, he told her he would teach her only if she would commit to that practise).

As a horn student (maybe I was around Grade 5 then) my teacher said she required one and a half hours a day but that may have been because I had a lot to catch up on. I kept with that amount of practise all the way from there up to post Grade 8 (ish - I was in the US then and they don't do grades but I judge it by the repertoire I was working on then).

While I disagree with my pupils who say they have no time, specifically "I can't practise on such-and-such a day because I have gymnastics", I can see that many could not fit in more than an hour's practice a day, even at higher grades, when they are also having a hour's homework and a private life to fit in. And if their dometic situation is such that other people are put out if they are to practise - those people whose piano is in the same room as the television and there's no way to alter this, and there are other members of the family who don't really want to spend more than an hour listening to someone practising the piano rather than watching TV (for instance) - they're rather limited not by their own willingness but by their lifestyle or that of their family.

Indeed - I was lucky that I was a horn student at university where there were practise rooms available 24/7. My friend lives with her partner only, so that was also not a problem for her. I do feel for people whose housemates or family object to them practising.