I'd guess the enchantments are just checked when an opponent first comes into range, and only re-checked if one or both characters move away and then back into range.
With my warrior character in the original release of D2:ED I don't recall poison auras being triggered multiple times with the same opponent, but then normal opponents usually didn't last long enough to get to another check if there was one 3 seconds later. I only tried Spontaneous Combustion once, and it didn't seem to be triggered as much as poison Aura (or the effect just wasn't as noticeable).
I didn't use area effect auras with my ranger in DKS. Since some enchantments were changed, my assumptions based on D2:ED may be incorrect for DKS even if they are accurate in D2:ED.
