how much time between eating and exercise
No relation with lunch , as such because lunch you will be having in afternoon and gym in evening. But if you are eating again before workout like carbs / pre workout stuffs etc it would be good to have 30 - 45 mins gap after eating. Reason is some people feel nauseous because the metabolic activity plus physical activity and high heart rate. To be on safer side have some thing before gym but a 30 mins gap will be good enough.