This depends on quite a few things
- which insulin you are using. For example NovoRapid is sometimes renamed "NovoSluggish". Fiasp works faster and then there are other insulins which I don't know about.
- how insulin sensitive you are. NovoRapid, for example is reported to last for between 3 and 5 hours. The duration tends to be a personal thing. I would say it last for about 3 hours for me but others experience the affects of it for longer than 5 hours.
- whether you have any fast acting insulin already "on board". So, if you injected a couple of hours ago, you have some insulin which is already working. Be waring of "stacking"your doses.
- what is the current trend of your BG. You may have a BG of 10.0 because it is coming down from 15.0 or because it is heading up from 5.0. This may be caused by your basal.
In other words, all corrections, like every other insulin dose, is an approximation.