All units and structures that require building, requisitioning or deploying have an amount of //time// that this takes. Recruiting an infantry platoon requires a certain amount of time, as does an armoured car, tank, or self-propelled gun.
These //construction times// are part of the larger organization of balancing that we work into the game. Generally, the better a unit is at something, the more time it takes you to get it. Some units may appear to be quite powerful under certain situations which may bring one to question why they are so quickly and readily available. If this is the case, then there must be some reason, some weakness that can be easily exploited to explain why a unit is so common.
When constructing, upgrading, recruiting, requisitioning or capturing, there will usually be at least one in-game indication of this, from obvious countdown timers to other methods that are easily recognizable and give a noticable hint to how long an action will take.