This Journal of Diabetes Science and Technology paper addresses the issue of insulin’s action over time. Rapid acting analog insulin’s duration is commonly thought to be in the 5-6 hour range.
Here’s a graph from that paper that shows the percent remaining insulin on board (IOB) with different duration assumptions.
Using this chart, if a five-hour duration of insulin action is assumed, here’s the approximate %IOB over five hours and the distribution per hour.
1 hour - 90% IOB or 10% used this hour
2 hours - 70% IOB or 20% used this hour
3 hours - 35% IOB or 35% used this hour
4 hours - 12% IOB or 23% used this hour
5 hours - 0% IOB or 12% used this hour