Or he could run into the nearest room with armed allies. You know, a guard running to alert other guards.
Sure... But he still needs to move there, and talk to the sergeant/low level flunky in charge of the force. And such a leader might indeed need to be persuaded to jump in there.
ROUTED MONSTER: "Boss, there's some adventurers in there?"
SGT: "Really? Everyone, get up, grab your gear! What's going on?"
ROUTED MONSTER: "Well, they've killed everyone!"
SGT: "Hrm. On second thought, let's go ask the Chief before we jump in there. I sure don't want to get slaughtered!"
I think there are three main scenarios with intelligent, tactical, organized occupants that maintain a QRF and are expecting trouble:
- Strong leadership, centralized QRF. This is most likely for lairs that have multiple access points to defend. The monsters will want their "reserve" to be able to get to any trouble spot easily, so it will be in the middle of the lair. Additionally, the leaders will want protection from raiders that utilize the multiple access points, so they'll want the QRF as bodyguards. This scenario easily rationalizes a 5 minute delay for transit time.
- Strong leadership, forward QRF. This is most likely for lairs that have a strategic strongpoint to hold. If there's only one access point, then I'd expect it to be well defended, and the QRF's job is likely to counterattack immediately to retake the main gates/access tunnel/teleport circle/whatever. The leaders are aware of this situation and ensure that the QRF is well led to quickly respond, although this exposes leaders to a lot of personal danger. This is the most dangerous situation as a QRF might well respond in <5 minutes. However, if the PCs are smart and find a way around the single access point the monsters will be in trouble! Additionally, the PCs likely have a chance to kill off an important leader early in the fight and then withdraw to lick their wounds.
- Weak leadership, forward QRF. I find this more likely than the above scenario. Most leader-monsters, being selfish and evil, probably don't want to be too far forward. Forward guard posts are boring, lack creature comforts, and are potentially hazardous. Instead, they'll likely be comfortable in the center of the lair. This scenario makes a delay quite likely as the forward QRF waits for the leaders to authorize its forward deployment, especially if the forward QRF fears being mauled by the party that just killed off all of the first batch of monsters!
I find it unlikely that disorganized opponents maintain a strong QRF. A disorganized foe would be likely to:
(A) Have a weak QRF -- perhaps orc warriors trickle out of the lair a few without any significant organization.
(B) Take more time to mobilize. By definition, a disorganized force is slower to assemble!
(C) Is not coordinated with the original monsters. Perhaps an opportunistic hydra jumps into the melee, attacking PCs and the original monsters alike without discrimination.
All of those scenarios are viable and create tactical options for the PCs to make it surviveable.