In T2 or T3 I've never had or seen a long queue that would take more than 10 mins. In T1 I have seen longer queues when 2 QF A380s and a 747 each from BA and QF have somehow managed to converge on SIN in a short window. But anything longer than 30 minutes would be a trigger for a national enquiry.

I'll go one up on SNA1K, once when my luggage didn't make a connection at LHR to SIN, in a hour and half I have cleared immigration, gone home, showered, packed some new luggage and returned to the airport, checked in, passed back through immigration and into the lounge waiting for my connecting flight to MEL......