Just as soon as they actually find something to indict. Given the current record, odds seem to be good on "Never."
See, that's the problem with state secrets. You cant say "See, this is a state secret, right here!" because the thing you will be pointing at will be a big wall of blacked out, redacted text that "Could be anything, really."
Unless the prosecutor is willing to out state secrets himself by refusing to allow redaction on court documents, it is impossible to create a case without using a secret court.
So, instead we have intelligence insiders, who DO have access to the unredacted data, saying "No, for realz yo, she broke the rules majorly!" and people poohpoohing it because "clinton 2016!" and "That could have been anything!"
Basically, people saying "Intelligence agency insiders aren't a reliable source" and or "Seriously, it's just a bunch of emails, what's the big deal?" and or, demanding a burden of proof that is undeliverable (again because Clinton 2016!), and in all cases, substituting their own opinion over that of actual intelligence agency experts.
If you accept the expert testimonies of intelligence agency employees working the scandal, clinton is dirty.
If you dont accept their testimony, "See, there is nothing to indict over!"
That really is what it boils down to. It is too unpopular to prosecute Clinton right now. At best, she will be indicted when the secrecy hold dates on those documents expires, 50 years from now.
She will probably be dead of old age by then.
So, indictment: never.