I've quite recently started flying into SIN and right now given the snow here, I much rather be at Boat Quay.

Anyway, the longest I spent before getting to the taxi queue was 10 - 15 mins, cos I think a few jumbos unloaded luggage/pax together or something.

the fastest was abt 2-3 mins and I was at the taxi ranks which work fast too.

The city itself is not too far from the airport, i guess a 20 mins cab ride.
There is a train system from Airport to the city but I have yet to try that though.

so 30 mins is plenty enough by SIN stds.

enjoy your stay and pls obey the laws over there, they are strict abt that.

All the best.