Long acting (12 hours to 24 hours) insulins are time released formulas. They should be just as effective an hour after your injection as when they’re in their final hour. If you had 24 hour insulin, you could do your injections in the mornings or evenings, but you wouldn’t want to overlap your doses.
Most diabetics who use insulin now use two different types of insulin, the slow acting you were asking about, and the fast acting which starts acting in about 15 minutes of injection and will peek in an hour to two hours after the injection.
You use the long acting insulin for baseline insulin needs and the short acting insulin to cover meals. This way, you act much more like a pancreas. In fact, fast acting insulins have gotten so fast that a diabetic can now judge their insulin needs while eyeing their plate.
This is much different from what it was 50 years ago. Back then, you only had bovine or pork derived insulin and they were all more or less mid-term acting. They would start working about a 1/2 hour after injection, peak at about 4 to 6 hours after the injection, and quickly fade away after that. Timing was of crucial importance. You had to guess what you were going to eat four to six hours in advance of your injection.
At the same time, blood monitoring equipment was non-existent or extremely expensive. Most diabetics used urine strips to measure their glucose levels. However, there is no glucose in the urine if your blood glucose level was below 200 to 250 mg/dl. The average fasting glucose is between 70 to 100 mg/dl with it peeking at around 140mg/dl. If blood glucose levels drop below 70mg/dl, you have low blood glucose, and if they drop below 50mg/dl, you can die in a matter of hours.
Thus, a diabetic had to keep their blood over 200 mg/dl because once it was below that range, they didn’t know if their blood glucose was 150 mg/dl or 40 mg/dl. The continuous high blood glucose levels damaged the nerves, heart, lungs and kidneys. In other words, if you were a diabetic back in the 1960s, your choice was a fast painful death due to low blood glucose levels or a slow painful death due to high blood glucose levels. If you guessed wrong with your insulin needs you could land up in the hospital.
Now, thanks to better insulins and better blood monitoring, diabetes is much more manageable. Many diabetics now keep their blood glucose levels in normal range throughout the day and experience fewer long term health effects.