Is there anyone that can tell me is it worth getting L mus for classical guitar in Australia or just 8th grade or A mus etc. If anyone has done the exam can you tell me what you have to do, what you did or any other preparation that you did. If you did it as well can you let me know your experience with it and how much practice you did overall.
It will depend a lot on what you intend to do with (or without) it. There have been discussions on this site as to the usefulness of the 'piece of paper'. Some intimated that grade viii was only a starting point as far as being a pro player is concerned - probably from a classical standpoint.
It also depends where you are in your playing standard, too. After grade viii, a lot of boards split stuff into performing or teaching - with quite different syllabi for each, not surprisingly.
There are myriads of players, professionals, in all sorts of genres, who don't have certificates to show their prowess. Their playing, thus reputations, does this for them.
Having said that, classical playing seems to attach a lot more importance to certification, rightly or wrongly. So, if the goal is to become a classical guitar player of note (sic) it quite possibly is the best way to go. It will depend on the board as to what you HAVE to get before moving on to the next step. Check out ABRSM, Trinity, LCM as some choices. Also one syllabus may suit you better, although actually taking the exam could be problematic in Australia - I don't know.
I'm personally a big believer in musical exams. At the very least they will teach people the self discipline of deciding on a ambition an working every day for many months to achieve it.
This type of discipline is useful to people in any field of study. If you do nothing else with your music it will at the very least teach you this.
Ultimately whether or not it is worth the time and effort is not something that an answer can tell you. For me these types of exams are rewarding because I see them as an affirmation of my skills and it gives me a sense of achievement.
Whether this would be the case with you is not something I can tell you.
In regards to how much work it would take. I will not think it wise to at least take a year to prepare for this exam. 3 - 4 hours a day 5 days a week. It will be long and difficult.
You can check out the specific syllabus to the ABRSM grade 8 course HERE!