The local gov't websites and other printed info re: PRC application are inaccurate. The available info makes it seem that once you have been married to a ROC citizen for 5 years while also having resided fulltime in TW, then you can apply for PRC. But that is not the case.
"Immigration Law 23" (which can be found at www.tmpd.gov.tw - Taipei Municipal Police) states in order for a spouse to apply for PRC:
"(II) Be the spouse or child of a Taiwan citizen, who has resided legally in the ROC for at least five successive years, during which the applicant spent at least 183 days each year within the ROC."
The key words are "resided legally." This means that they count 5 consecutive years of your ARC that says you are joining family, and not the 5 years that you may have spent already on a visitor visa/work ARC, etc.
In my case, my first ARC was for work and they won't consider it, even though I got married a year before I got my work ARC. I got married 3/96, and got my join-family-ARC in 8/98, so they will start counting my 5 years for PRC beginning on 8/98.
So in other words, as a foreign spouse, you must have "ARC joining family status" for at least 5 years to apply for PRC.
In the meantime extending an ARC only requires a marriage and house certicate and a $1000 to $3000 fee. No health test or citizen records. The ARC can be extended ad infinitum like this with no limitation and seems to be less of a hassle than compiling all the docs for PRC. It also takes about 10 minutes to extend.
Hope this info helps.