Just to add to the knowledge bank here, two recent success stories:
1) Single Male, 38, earning 4k pm, been in Singapore 15 years. Applied PR in November 2014 and received AIP in Feb 2015. Malaysian Chinese.
2) Single female. 45yrs old, been in Singapore 35yrs, PR for 30yrs. Earning >20k pm. Applied for citizenship in Dec 2015 and received AIP in April 2015. Also Malaysian Chinese.
We know there is a higher chance of success for Malaysian Chinese candidates, but I am also of the opinion that a lengthy commitment to Singapore prior to application increases chances. More so than industry, earnings, family, home ownership. It also increases chances of a faster response time, it seems.