You are referencing the stun related to BASH which does not have a duration, just interrupts.
This is stun related to spell effects, which does indeed use the effect duration.
Code:
case SE_Stun:
{
#ifdef SPELL_EFFECT_SPAM
snprintf(effect_desc, _EDLEN, "Stun: %d msec", effect_value);
#endif
//Typically we check for immunities else where but since stun immunities are different and only
//Block the stun part and not the whole spell, we do it here, also do the message here so we wont get the message on a resist
int max_level = spell.max[i];
//max_level of 0 means we assume a default of 55.
if (max_level == 0)
max_level = RuleI(Spells, BaseImmunityLevel);
// NPCs get to ignore max_level for their spells.
if(SpecAttacks[UNSTUNABLE] ||
((GetLevel() > max_level)
&& caster && (!caster->IsNPC() || (caster->IsNPC() && !RuleB(Spells, NPCIgnoreBaseImmunity)))))
{
caster->Message_StringID(MT_SpellFailure, IMMUNE_STUN);
}
else
{
int stun_resist = itembonuses.StunResist+spellbonuses.StunResist;
if(IsClient())
stun_resist += aabonuses.StunResist;
if(stun_resist <= 0 || MakeRandomInt(0,99) >= stun_resist)
{
mlog(COMBAT__HITS, "Stunned. We had %d percent resist chance.", stun_resist);
Stun(effect_value);
}
else {
if(IsClient())
Message_StringID(MT_Stun, SHAKE_OFF_STUN);
mlog(COMBAT__HITS, "Stun Resisted. We had %d percent resist chance.", stun_resist);
}
}
break;
}