That would depend on many factors such as: assuming you're on a basal/bolus insulin regime is your basal insulin correctly calculated to keep your bg steady throughout the day (fasting tests can help to determine this), when last did you inject fast acting insulin and how much did you inject, did you correctly match your insulin to the amount of carbs you ate, what type of food did you eat at your last meal (some foods have a lower gi than others), how quickly insulin lowers your bg is different for everyone as some people are more sensitive than others, where you injected insulin can alter how quickly it works, different insulins have a different time profile so depends on what fast acting insuling you're using, time of day can also have an impact on this as some people find they're more insulin resistant in the mornings compared to afternoons.