You need to think about how quickly or slowly the insulin acts. If your pancreas is making insulin, it will be releasing it directly into the bloodstream, where it can act immediately. When you inject insulin, it sits around in the subcutaneous tissue near the injection site and is gradually absorbed into the bloodstream. Basal (long acting) insulins like Lantus are designed to take around 24 hours to be completely absorbed - so one injection per day will cover. Bolus (rapid acting) insulins lke Novorapid or Humalog are designed to be completely absorbed in about 5 hours. They are designed to match the speed of digestion of food. the Therefore 10 Units of Lantus will take much longer to bring down your levels than 10 units of Humalog.but it will do so eventually.
To add to the information about the use of the C-peptide assay: C-peptide is a by-product of the normal production of insulin by the pancreas - it is not present in injected insulin (which is manufactured in genetically engineered bacteria or yeast). Assays to measure insulin levels in anyone taking insulin by injection will not distinguish between injected insulin and insulin still being made in the pancreas. C-peptide can only have come from the pancreas, therefore it is a good measure of insulin production.
Joel