2 Hours After Eating

It depends what you have eaten, how many carbs were in it, how long before eating your had your insulin, ...

You should be able to calculate an approximation - assume half of the bolus has gone after 2 hours.
However, bear in mind if you ate something fatty (e.g. pizza) it is likely to take longer than 2 hours for the carbs from that meal to affect your BG.

Some people (especially those with CGM or Libre) keep records of what they eat and how there BG reacts over the next few hours. Using these records they are able to work out the best time to inject.
For example, if you find pasta takes longer to affect your BG, you can inject later whereas you may find potatoes has an instant affect so you should inject earlier.
But everyone is unique: this is about how your body reacts to the food.