Taiwan foreign policy operates on a principle of strict reciprocity, so the e-Gate membership for GE holders lasts as long as GE membership for Taiwan citizens (i.e., 5 years); this is also why both cost basically the same, both require an in-person interview with stupid questions, etc. etc. For whatever reason, the validity period doesn't currently appear to be listed on the NIA website, but I can confirm that my membership expiration date is 5 years from the date of my e-Gate interview.