How quickly do ovens lose heat when they are opened?

It's going to vary with the construction of the oven. More metal => more thermal mass => slower temperature change. read more

These objects act as thermal mass, and account for the vast majority of the heat in the oven. The air, by comparison, is much less dense, accounting for a small percentage (my guess is <10%) of the heat in the oven. read more

Heat is also absorbed by the metal liner, racks, cookware, baking stones, and food. These objects act as thermal mass, and account for the vast majority of the heat in the oven. The air, by comparison, is much less dense, accounting for a small percentage (my guess is less than 10%) of the heat in the oven. read more

While the door is open many modern ovens will not turn the burners on so if you leave the door open for an extended time the residual heat may not be sufficient to quickly maintain heat and the burners will kick on to return the air temp to its proper setting. read more