Heat lamp and food for 3 week old puppy

Hello, at what age do I stop using the heat lamp for puppies and how many weeks do they naturally eat the paste? Thank you for your answers This is just so as not to stop the lamp if necessary and to prepare for weaning They are 3 weeks old, they are pomchis crossed chihuhua and dwarf spitz For the moment still using mother's milk but I want to be sure not to start too early