Ah yes, requests for medpacks/ammo. I tend to respond to these within one second, usually less. There are times when a marine calls for a medpack maybe after one bite, or two. Sometimes they call a bit earlier, then I am ready to drop them whenever needed.
Order requests, I go to these within 2 seconds. Medpacks and ammo first, though.
During a siege or other important time, I still respond to these calls quite quickly, as they are still just as vital to the marines during that time as any other.