Reviving this thread with a recent data point of US Global Entry member applying for Singapore eIACS:

- completed an online application last August, received an Approval In Principle within a day or two, but it also said to visit an enrollment center within 2 months to complete the process. However I wasn't going again until January (this past month)

- submitted another application in late December, again received AIP within a day or two. Note that some time in between August and December, they relaxed the requirements so that you only need to have entered Singapore twice in the last 24 months (reduced from 3x within 12 months, although I did qualify in August under the stricter criteria)

https://www.ica.gov.sg/enteringanddeparting/visitor/ftp

On arrival into Changi, I remained "airside" and went to the office in T3 to complete the process, which was quick and painless (the only extremely minor annoyance was having to fill out a paper application containing mostly the same info as the online application). I was advised it could take maybe 15 minutes for my approval to percolate through the system, so I sat around reading email etc. for a bit and was then able to use the eGates with no problem ^