I'll wager a guess of under ten minutes, all my modern cars have warmed up in about seven minutes and the carb'ed cars weren't a whole lot slower. This is while driving ~50mph, it will of course warm up slower at idle.
Does the choke need to be on for that long though? Mine has a manual choke and I use the choke for a whopping 30 seconds or so and seems to work for me. I'm curious because I'm switching out the eddy manual choke carb to a holley street avenger electric choke. I'll be crossing this bridge soon as well.
Red takes about 45-60 seconds to warm up. Did you try to kick it down by pressing on the gas pedal during high rev warm up? I normally let it high idle for the above 45-60 seconds, then just kick it down. Ready to rumble.
Ambient temperature will have a fairly significant impact. The real question is how long does it stay on versus how long it needs to stay on. The purpose is to allow the car to run smooth until it warms up. The car will still run fairly smooth with the choke on after it's warm, but you're burning excess fuel. Your choke can come off in 1 or 2 minutes if your engine will run smooth. You almost have to play with it. My 67 has an automatic choke that has always worked well. The 68 has a manual choke. I really prefer the automatic because you just don't have to remember to pull the choke to get the car to start. Ask me how many times I've forgotten and just cranked and cranked.
I did get the car running pretty well. The choke does open up as it should and it is running strong. I'm still getting a little bit of the popping of the exhaust after the choke opens fully, though. It's not as bad before. I'm guessing a little more gas might not hurt it. I'll keep you all updated.
Keep in mind that the choke's bi-metal spring is not strong enough to pull the choke butterfly open while the fast idle screw is resting on the choke linkage cam. That's why you have to "kick it out" ... it momentarily releases the tension on the fast idle screw caused by the throttle return spring.
In other words, if you don't touch the throttle after the choke has been set, you can let it idle for a year and the choke will not open.