You're misunderstanding what SemiTether is.
There are only two types a jailbreaks; Tethered, which requires you to boot in a special way every time you shut down, and untethered, which allows you to boot normally.
SemiTether is a tweak that affects tethered jailbreaks (not a jailbreak in and of itself). It makes it so that instead of needing to connect to a PC and upload a package to your device in order to boot up, you can boot in a different special way, one where you are essentially un-jailbroken, but you can at least boot up without the aid of a PC. SemiTether is maintained by a group of tweak developers from TheBigBoss repo, not by any of the dev teams who actively work on jailbreaks.
SemiTether is not a jailbreak. SemiTether is a tweak that jailbroken devices run.
I should point out that the only reason iOS 6 was jailbroken so close to release (it was actually jailbroken on day 1, and so were each of the beta releases) is because of limera1n, a non-patchable bootrom-level exploit that all devices with A4 and earlier processors are vulnerable to. No matter what Apple does software-wise, the iPhone 4, iPod Touch, and iPad 1 will be able to be jailbroken on any firmware whatsoever with the devs having to do very little work. limera1n, however, will always result in a tethered jailbreak without additional exploits to untether. In fact, limera1n only frees the file-system. There is still work that must be done afterward to make things like Cydia work.
Unfortunately, ever since the release of the iPad 2 and the A5 processor, the limera1n exploit has been patched, so there is no public bootrom-level exploit that can affect any device released after the iPhone 4 and iPod Touch 4G. Without limera1n being able to take out the bulk of the work all on its own, dev teams now have to find more exploits with each new iOS release just to get a working public jailbreak. Even more bad news, a lot of the devs have been talking about just how much Apple has beefed up security in iOS 6, (and seemingly in iOS 6.0.2 which is exclusive to the iPhone 5 and iPad Mini) and then even moreso in the betas of iOS 6.1, even though a public jailbreak hasn't even been released. I hate to say it, but the future looks grim for jailbreaking iDevices. As always, it will happen eventually, but eventually may be a lot longer than we're comfortable with.
iOS 5 was fully jailbroken approximately 3 and a half months after its release. 5.1 was jailbroken in 2 and a half. iOS 6 has been around for about 2 months now, and with the devs talking about all the added security, it would be reasonable to say it'll be at least a month, probably more, but devs do like to surprise us, so you can never know.