It all depends on whether you are baking a whole cauliflower or using cauliflower florets. A whole cauliflower will need 40 minutes or so to bake fully until it is fork-tender. Cauliflower florets need 25-30 minutes of baking in a 375-400 F preheated oven. It is always best to use your fork to check for doneness. Also, it cut down on baking time, consider steaming the cauliflower for a few minutes in some boiling water before adding it to the oven. This is certainly an option and not necessary.