the last ones that came out were pretty recent. like 2 months ago maybe? the hush-hush achievements come out rarely. i bet it'll take another at least 6 months until the next ones come. how do i know it? i don't, it's just experience from how it went the last few years.
no one knows when new achievements will be coming out. ocassionally they spoiler it in the news on the front page.
https://osu.ppy.sh/home/news/2018-12-24-new-featured-artist-camellia for example. "In addition, some founding members of the Mappers' Guild have been hard at work creating their take on some of the new tracks, which you can check out below (though do note, these maps aren't ranked yet)" so it's pretty likely that we get a camellia theme pack or a new mappers guild pack next and personally i think it will happen soon.
if you mean information on what you have to do to get them:
http://haitai.jp
there used to be a thread in the suggestion forums where you could give ideas but i think they got rid of it. at least a few months ago i wanted to give an idea / suggestion and it was gone.