According to user feedback and testers, for a marathon, you want to eat a packet every thirty to forty-five minutes. The human body can digest about 350 calories an hour, so figure that two single serving one hundred calorie packs will be exactly what you need. They will start to work immediately to make sure that you keep an eye on your time.
Each product will carry instructions indicating the amount you can consume and how often and these instructions are designed to be exactly right for those in a competitive or challenging activity but after the activity it is a good idea to switch back to natural food sources to maintain your body and only use the  energy gels for the purpose for which they are made.
If you make the gels part of your daily regular diet they are good for getting energy but they do lack other things which we get naturally from our daily natural foods and in order to maintain a healthy body we are dependent on natural rather than manufactured food sources. Like all foods, it is important that we consume them in context.