As with all insulin preparations, the duration of action of Humulin R U-100 is dependent on dose, site of injection, blood supply, temperature, and physical activity.
Humulin R U-100 is human insulin with a short duration of action. With subcutaneous use, the pharmacologic effect of Humulin R U-100 begins approximately 30 minutes (range: 10 to 75 minutes) after administration of doses in the 0.05 to 0.4 units/kg range. The effect is maximal at approximately 3 hours (range: 20 minutes to 7 hours) and terminates after approximately 8 hours (range: 3 to 14 hours). With intravenous use, the pharmacologic effect of Humulin R U-100 begins at approximately 10 to 15 minutes and terminates at a median time of approximately 4 hours (range: 2 to 6 hours) after administration of doses in the range of 0.1 to 0.2 units/kg. The time course of action of any insulin may vary considerably in different individuals or at different times in the same individual.
Taken from here
Humulin R - FDA prescribing information, side effects and uses