The short answer is this: promotions are given when they are deserved and warranted.
The big promotion lists usually appear every few months, but it varies based on the division. Ingame actions, Ventrilo activity, and forum activity all play a role in such decisions, but I will put it this way: to be promoted, someone needs to believe that they can trust you in command of a server, at least to some extent. Additionally, it is to your incredible benefit to have a functional microphone, and I think the reasons for this should be obvious.
If we don't see you (especially) ingame, how are we to know who you are? In DoD, for example, we have a large number of members that hang out in Ventrilo every night, just because it's what we do. Many of them are eGOs; it's not that we promote people for being in Ventrilo, but the decision is much easier when you can talk to people and get to know them to some extent. There needs to be a foundation of trust for a group like this to function properly.
We joke occasionally that Rules 1 and 2 should never be broken, except when congratulating somebody else, so I'll restate them just because it's the first thing I think of when I see threads like this:
Rule 1: You do not talk about promotions.
Rule 2: You DO NOT TALK about promotions.
To summarize: They're given out when they're given out, there's no set schedule, and are based on maturity and activity.
Answered, thread locked.