Just a reasonable amount of time to allow people to get help updating, a relatively obscure function like this is probably ok to remove quicker than normal, a more common function might be left longer than normal. In any case it was mentioned briefly in [the original email](https://mail.mozilla.org/pipermail/rust-dev/2014-April/009543.html), specifically

> we'll likely leave in #[deprecated] functions for about a month or so.

In terms of actual implementation details of removal, I personally think just using occasionally checking for things to remove & consulting the git history for timing is good enough (since we don't have so much code/`#[deprecated]`-churn that doing it manually with `grep` will be particularly hard); others may disagree with this though.