It took me a year to do violin diploma and 9 months for my viola diploma afterwards. It usually takes much longer and I have a friend who started his violin diploma 3 years ago and still is nowhere near ready! I have also done my violin LRSM (licentiate of royal schools) and just did my FRSM (fellow of royal schools) a few months ago. It took me 2 years and 3 years respectively for LRSM and FRSM.
The good thing about Diploma is the post nominal letters! They are such a novelty to have- especially now that I have 3 from ABRSM even before having completed any proper qualifications apart from GCSE's! I don't think that you get these for Jazz, but it all depends what you want to do, if you are thinking of going to a conservatoire I would strongly recommend Diploma. Plus it really is great to lord your letters over people!
Lord Harold Dip.ABRSM LRSM FRSM !!!