I completed the enrollment last Friday at Changi Airport. It was very simple with no waiting and no actual interview. The office was airside in T3. It took about 15-20 minutes and probably could have been faster but I had the feeling the two officers were doing it for the first time for US GE members. They took my thumb prints and scanned my passport 2/3 times. They stamped the last page of my passport with a notice stating that I was eligible to use the elACS and told me that even if I go through regular immigration, I could just show the stamp. Each visit was good for 90 days.