How Often does the Xbox 360 Ban Wave Happen?

I'm told that, when you get onto Microsoft's ban list, you aren't immediately cut off - Microsoft will allow you to play for some time (presumably to get you hooked), and then execute a wave of bans across the entire network.

How often does this happen? Is there any particular formula for the wave's frequency, or does it only happen when X number of people achieve bans? Or is it completely random?

I don't think the ban wave is something you can predict by date. Keep in mind that Microsoft isn't just banning any player, they're specifically banning players that they can reliably detect mods on.

Mods vs. Microsoft is a back and forth game. The community releases a mod that is "undetectable" and some time later Microsoft develops a way to track it. Sometimes it can be tracked with current software, other times it requires a mini dashboard update before Microsoft can detect it. But they will eventually find a way to detect it.

Once that system is in place, Microsoft starts collecting accounts to ban. Traditionally, Microsoft has held on to these accounts until just before a big launch, as was the case with Halo: Reach. Maybe they think banning right before a big game is going to get you to go out and buy a new console?

However, there's no guarantee that Microsoft will ban people before any given game release. It depends on the cycle: is there a Mod that is gaining in popularity? Has Microsoft had the time to develop a reliable detection method for the most recent mod? No one knows for sure, but the best way understand where we are in the cycle is to keep tabs on the modding community forums.

Microsoft normally processes the list on a particular day of the month. No one knows for sure when, but I would guess it would be early or late in the month.

They used to have a detailed help post on the X-Box website with the details, but it was removed during the last revamp. I am guessing they don't want the details leaked to avoid abuse.