It really depends on the time of day/night that you are arriving...

There are 3 terminals in Singapore Changi Airport. Each terminal has two immigration exits with about 12-15 counters. The officers are friendly and there are even candies on the counter for you to take!

When 2-3 A380s and 747s arrive at the same time, you will be in a queue. I have been in a queue for sometimes over an hour if a few aircrafts land at the same time.

But generally, even if you exit with the economy class passengers, you will never spend more than 20 mins in a queue and by the time you reach the luggage belt, your bag would most likely be there.

Only thing to remember is that if you are carrying cigarettes even if you bought them duty free, they are taxed in Singapore and it is best if you declare them at Customs.