Any time six months or more after the last status check is OK: see the "read before posting" post.
Note that asking here doesn't affect when the site will be reviewed. (I haven't figured out what people actually do with the information we give them, but it's not for lack of trying.) It won't hurt your site for you to never ask again -- in fact, it wouldn't have hurt for you not to have asked the first time.
I emphasis this because there is almost never a "SHOULD" ask -- there is seldom a practical or ethical imperative involved, and if there is, we'd have told you about it. There's only a "may" ask.