I'm not planning anything. Just curious.
I think the mods and management here probably wouldn't want to divulge that, likely for the same reason that they don't particularly want to reveal an exact set of rules that cause binning: People will abuse the knowledge.
If it's gray and fuzzy, they have the ability to act in accordance with what they feel are the best interests of the Trilogy sites. If they're concretely laid out, there would be a world of butt-hurt whenever they'd have to do something exceptional -- like a premature ban, or not being "stern enough" with a given user, or being "too stern" with another, etc.
The flip (and down) side to this is the standard lack of transparency that has, in the past, caused its own world of butthurt here on Meta. Frankly, mods and management of any Internet-based user-content area are likely to be in a no-win situation simply due to the nature of the Internet's various denizens.
I'm just reporting it how I see it -- in the words of Dr Manhattan, without condemning nor condoning.
In this answer, Jeff indicates that after the third suspension you're skating on very thin ice.
Largely by definition, the moderators are human (well, mostly) exception handlers, but unlike code there is no single rule that fits all cases. Really it simply boils down to common sense (or as close an approximation as we can get).
The specific incident / incidents is a big factor, as would be the ongoing history with that user; the majority of stuff is just quietly "dealt with" without resorting to any other measures, but without meaning to sound like we harbor grudges this quiet background noise would probably form a part of any warning message(s) sent to the user.
Taking that further, if you are repeating the same behaviour that has already got you into disrepute, then that is likely to be met with less patience (but please don't take that to mean that you can safely be obnoxious in entirely original ways and keep out of trouble).
Add to that; successive suspensions tend to be significantly longer in duration...
Another important factor is your history on the site; if (at the worst case) you have a 1-day old account with no e-mail address and your first contributions are defamation, spam, abuse etc - then you probably shouldn't be surprised when your account stops working without warning or delay.
Theoretically it's possible, but I suspect the practical reality is it would never be used:
They currently do an increasing ban length - a few days, a week, a month, a few months, several months, etc. In theory one could reach something like an 80 year ban, but it would take so many smaller bans that it's already been a decade before they hit that point. So yes, it's possible, but it's unlikely that someone would have enough staying power that after, say, a 2 year ban they'd be back the instant it ended and start causing a ruckus.
Assuming they follow a nice geometric increase, say powers of 2, and increase the time starting from one day, then the first ban would be 1 day, second 2 days, third 4 days, fourth 8 days, etc, then the following would be true:
The 8th ban would result in a 256 day ban, but by that time they've also served 128 days, 64 days, etc, and would have served a total of 511 days. This is over a year, so it would take 8 bans to serve over a year.
The 15th ban would result in a 44 year ban, but cumulatively they've already served nearly 44 years, so the 15th ban would end 89 years AFTER their first ban, which for all intents and purposes would be a lifetime ban.
They don't use this progression, though, and while they have the history of bans they've done before and generally try to be consistent, new bans are performed on a case by case basis.
But in most of Jeff's ramblings on the podcast and internet about problem users he appears to subscribe to the belief that anyone can change. I suspect that he would never hand down a "You can never ever in a million years come back so help me!" ban.
If it were me, I wouldn't do it because it's too much work to keep track of idiots for that long.