Looks to me like you're in for a 3 year ban,
but you better check that yourself and take a look at the paperwork you got.
This law firm has been so nice to provide a translation of the "Operation Directions for Banning Entry of Aliens (禁止外國人入國作業規定)". You should really have a look at it yourself, find the neat PDF here. There is also a slightly older version on the website of the Ministry of the Interior.
You don't tell us all of your story, but adding to overstaying for six years and three month detention I will also assume you worked illegally in the meantime. I find the following provisions that fit for you at first glance (emphasis mine):
An alien who has been sentenced by a court to detention in prison or a criminal fine shall be banned from entry for 3 years.
an alien who has overstayed for 1 year or more shall
be banned from entry for a time period equal to the period of the overstay with the maximum period of ban on entry being 3 years.
An alien who has worked illegally in Taiwan shall be banned from entry for 3 years
Note also in the same §
The period of ban on entry under the preceding paragraph shall be calculated from the day following the alien's exit from Taiwan.
It also helps to pay your fines if you have any open, as otherwise your ban gets extended by another 3 years (§6).
I do not find any mention of these bans being cumulative but let me repeat one more time: check the documentation you got, the stamps in your passport, the paperwork they gave you!
Disclaimer: I am not a lawyer and much less do I have any idea about Taiwanese law. I have no idea about the accuracy of the linked and cited translation nor whether that is the most recent version of the directive.