According to the owner's guide, "WARM is ONLY for keeping cooked food warm. DO NOT cook on the warm setting. We do not recommend using the WARM setting for more than 4 hours." Sadly, this does not answer your question. I don't know if the WARM if it shuts off automatically as we have not needed to keep food warm that long. Hope this helps.
you can set the crock pot for up to 20 hrs,I don"t know why you would. so apparently the warm tem. will stay on as long as the crock pot is on.the owners guide suggest you only use the warm temp. for no more than 4 hrs