Infantry Command has no effect on patrolling npc experience gain. The only thing it does is increase the amount of squads, and therefore soldiers, you can have patrol at one time.
If you want to increase the efficiency of npc training you can do one of two things. The first is to use the squad and spend your time hunting which allows for a much higher experience gain rate than patrolling but it only affects one squad at a time. The second is to build a maze for your squads to patrol through. By having a very long maze you increase the distance a squad needs to travel before the game checks to see if the squad stops patrolling or continues, which helps patrolling squads to level faster when compared to a squad that is patrolling a terrain square with no obstacles.
It depends entirely on how many tiles the squad has to walk through to reach the end of the maze and how many times the squad stops at the end of each patrol repetition. When there are no obstacles a squad will travel 20 terrain tiles before reaching the end point of their patrol route. Once an end point is reached the game will "roll" to see if the squad will stop for a few hours or continue patrolling.
The following requires a little math but it will give you a rough comparison between a maze and non-maze.
If you want a rough idea go to the city designer and use walls to create a maze. Count the amount of tiles the squad has to go through to reach the end. Have a squad patrol for 20 terrain tiles, which is the distance traveled for non-maze squads, and make note exactly how much experience they receive at the end of a single repetition and how much time it takes them to travel that distance. Divide the time and experience by 20 so that you know the experience they gain per tile traveled and the amount of time it takes them to travel that distance.
Multiply those numbers by the amount of tiles the squad has to travel to reach the end of the maze you created in the city designer to find out how much experience they gain for each maze repetition and the amount of time it takes to complete a single repetition. A level 10 NPC requires 45,000 experience so divide 45,000 by the amount of experience you gain from a single maze repetition to find out how many repetitions they need to complete to reach level 10. Multiply the time required to complete a single maze repetition by the amount of maze repetitions required to reach level 10 to find the total amount of time required for a squad to reach level 10 using a maze without stopping.
When a squad stops they stop for an hour or so each time so if you assume that 50% of the time they will stop then you can figure out how much extra time to add to the total amount of time. This can be done by dividing the total amount of maze repetitions by two which will represent the amount of hours you add to the total time above. This should give a rough comparison between maze and non-maze training.
It's about 75 minutes on average if you take the average time per rest and then multiply that by the average amount of rests per stop
Given the stop time it can vary massively (seriously, I've seen variations of several weeks), but in my experience going from level 1 to level 10 in a perfect maze setup is about 6 months of patrolling.