I'm no GD veteran but from what I've heard, the 'mini demons' which were previously hosted at various weekenders and open days over the year, are (very sadly) no longer running, probably meaning the two Golden Demons that do run a year will be at Warhammer Fest UK (see another thread for that talk) and Warhammer Fest Europe (held in Germany last year I believe).
I managed to join the Golden Demon hype just in time for the last Mini Demon in November, but unfortunately it might be the case that the WHFs are the only opportunities now.
Having said that, Warhammer World open days have their own painting competitions now (not quite the prestige carried with GD however). So if you're itching for competitions to practise for the big boy Demon, and get feedback, the open days are your best bet. You can keep up with the announcements of these events on the Warhammer World facebook page, and probably the Warhammer Community Facebook Page, and also the 'Events' section of the Warhammer world website.
Hope this is helpful, and if anyone spots I've said anything incorrect of presumptuous then just let me know.