Yes, timings will definitely vary. I timed how long my oven took to preheat with a cold dutch oven, and it was around 20 minutes, so that’s about how long I add

Yes, timings will definitely vary. I timed how long my oven took to preheat with a cold dutch oven, and it was around 20 minutes, so that’s about how long I add. But I’ve found the timing is not super critical, the main thing is not to undercook. (Note that most recipes say things like “check for doneness, and bake for another 5 minutes if the loaf doesn’t sound hollow”.)