It depends on what you eat, how much……
If you have eaten Sweets/Chocolates (Sugar), then, Blood Glucose (BG) Spike could happen in as little as 10-15-20 minutes.
Simple Carbohydrates might take a while longer (but not too much longer…maybe at 30-45 minutes or so) to cause BG Spike.
And likewise, various kinds of foods take different times to cause BG Spikes and this can vary person to person and also can vary at different times for the same person.
Proteins take a lot longer to affect BG. Fats (by themselves) cause Little to no Increase in BG.
Factors such as how Stressed a person is, What kind of meal they had previously and When and How Much they ate, are they Diabetic, and if so, then, for how many years, how Active/Sedentary the person’s Lifestyle is…can all play a role in when the Blood Glucose Spikes after a meal.
The American Diabetes Association (ADA) recommends testing BG at 2 hours after a meal and a value of 180 mg/dL or lower is considered normal.
However, more rigorous studies state that you do not want Blood Glucose to ever go more than 140 mg/dL, as there is a good Correlation to Heart Disease, if this happens often (BG goes over 140 mg/dL regularly).
If you not wearing a Continuous Glucose Monitoring (CGM) device, then, as discussed above, the more Sugar/Simple Carbohydrates that a meal contains, the earlier the BG Spikes.
One way to check this, if you are not wearing a CGM device, is to check at approximately 15 minute intervals. So, if you have eaten a big Chocolate Bar let’s say, then check at 15 minutes, then at 30 minutes and maybe at 45 minutes. You will then know where the BG trends up and when it starts to come down. The highest value from these tests could then be taken as the (approximate) Spike (Highest) value.