It's been awhile since I played SoC, but I seem to remember enemies respawning after 1-2 in-game days. Only the random enemies though.
I've not tracked any pattern, but, for ex, the military that guard the first exit out of the initial valley in Cordon (the one with the train tracks on top, and the crashed rail carts) respawned very randomly.
I cleared the initial 5-6 soldiers, then I traipsed that path for days on end, to sell everything I was picking up, and no respawns. Then I started to see just one of the soldiers keep getting respawned, for a couple of times.
Finally, after I was way past that point in the story the game respawned the whole 5-6 soldiers every time, so I stopped killing them.
The bandits, dogs and bloodsuckers in Wild Territory (the railway area I need to pass to get to Yantar), those seem to respawn every other time I'm there. Plus the game also seems to like to spawn random quests in these areas, asking me to kill the bandits or stalkers in whatever camp. Quests that I'm definitely not picking up anywhere; they just trigger upon entering the area.
The zombie stalkers in Yantar, I cleared a path of them to the X16 lab, then found out I've not got enough armor and walked the path back to Wild Territory. And a bunch of the zombie stalkers and the those annoying gas-masked ones that walk on fours were on me again.
Speaking about the X16 lab, I loved the audio effect they enabled when you get close to the lab, due to the whatever emission device that's supposedly brainwashing stalkers that don't come equipped with whatever headpiece I now have that makes me immune. The audio of the whole game is muffled and I just barely make-out my gun shots. It's pretty cool sounding and very immersive.