Not very. For example, at the company/startup I left recently, lunch would arrive at 12 and would sit out until the evening. So come 5 or 6 PM I would eat dinner from the lunch leftovers. Regularly consisted of different meats as well. I never had any issue, and have never had any issue with food (i.e. food poisoning). Not sure if I have a strong stomach or the general rules are overly protective.