Your in PA, I'm in southern Ontario, We probably have similar temps in the winter. I find it takes about 5 min. like Camoit said. That's 5 min of fast idle in the driveway without driving. If I don't let it have warm up time my 89 takes an eternity to warm up. If your thermostat is good and your coolant level is good then another thing to check would be the air control valve. It should be making the breather pull warmed up air from the exhaust manifold shield when idling. Also check the flex pipe that connects the breather to the shield to make sure it's not blocked. The fresh air pipe should not really be an issue as anyone with a header can tell you.