A bake oven is a very big pile of masonry. Get all that thermal mass up to temperature, and it will fluctuate fairly little, and occasional stoking with a little wood can keep it hot. This is why baking was commonly done either by bakers, all day/night long, or by housewives one day a week: once the oven was hot, you wanted to make full use of it. The alternative for housewives was a Dutch oven, a deep covered cast-iron pot that could have coals placed on the lid, that could be used in the fireplace. But it couldn't do loaves of bread and pies as well as a bake oven.

For how to judge the oven temperature, here's what Lydia M Child said, in her The American Frugal Housewife :

Heating ovens must be regulated by experience and observation. There is a difference in wood in giving out heat; there is a great difference in the construction of ovens; and when an oven is extremely cold, either on account of the weather, or want of use, it must be heated more. Economical people heat ovens with pine wood, fagots, brush, and such light stuff. If you have none but hard wood, you must remember that it makes very hot coals, and therefore less of it will answer. A smart fire for an hour and a half is a general rule for common sized family ovens, provided brown bread and beans are to be baked. An hour is long enough to heat an oven for flour bread. Pies bear about as much heat as flour bread: pumpkin pies will bear more. If you are afraid your oven is too hot, throw in a little flour, and shut it up for a minute. If it scorches black immediately, the heat is too furious; if it merely browns, it is right. Some people wet an old broom two or three times, and turn it round near the top of die oven till it dries; this prevents pies and cake from scorching on the top. When you go into a new house, heat your oven two or three times, to get it seasoned, before you use it. After the wood is burned, rake the coals over the bottom of the oven, and let them lie a few minutes.