It stays in the blood between 20 and 24 hours. Some insulins come mixed together. For example, you can buy regular and NPH insulins already mixed in one bottle. They make it easier to inject two kinds of insulin at the same time. However, you can't adjust the amount of one insulin without also changing how much you get of the other insulin.
Insulins come dissolved in liquids at different strengths.
Most people use U-100 insulin. Be sure that the syringe you use matches the insulin strength. In Europe and Latin America, U-40 insulin is also used. If you're outside the United States, be certain to match your insulin strength with the correct size syringe. All insulins have added ingredients to keep them fresh and help them work better.