Craig is right. The check valve in the monovalve was deliberately designed into the system to prevent hot coolant from getting into the heater core when the engine is stopped. If the check valve leaks, that's another question.

Even with a perfectly working system, the ducting still will absorb a good deal of heat from the engine, the ambient air, and the sun. The more the car sits, the more heat will be absorbed. When you return and start the car, the a/c system has to remove all of that heat before you get cool air in the car. There's not much you can do about it, either, except park in the shade, cover the car, move to Alaska, etc.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I actually replaced my entire monovalve when I started to get a blast of heat after restarting the car in the summer because I assumed the check valve was leaking or sticking. It solved the problem, but I don't know if the check valve was really the problem or I would have gotten the same result with just a rebuild kit.