If there's a fairly long exhaust run after the muffler, I usually go for 1-2 gallons after I see color. By the time I've got color at my engine exhausts I've already had (diluted) antifreeze coming out of the muffler for at least a gallon, more likely 2.
For my engines (not Lehmans) with huge 2 stage water lift mufflers they get 9 gallons each. For comparison, the much smaller cooling system and muffler on the generator gets 4 gallons (there's color out the exhaust by 3 gallons and much less water in the muffler to cause dilution).
Keep in mind that freeze point and burst point aren't the same. The -50 antifreeze will freeze around 12* F, but it won't expand enough to hurt plastic plumbing, mufflers, etc. until about -10* F. Of course that's undiluted. So what antifreeze to use and how careful you need to be about dilution will depend on the winter temperatures you experience.