Code below, but it looks like 7 seconds and it makes a sound when you make the special attack but not if you fail to do so within the time allotted.
From buffs.xml:
<buff name="buffPerkBarBrawling8Complete" name_key="perkBarBrawling8CompleteDesc" description_key="perkBarBrawling8CompleteLongDesc" icon="ui_game_symbol_7th_curse">
<stack_type value="replace"/>
<duration value="2.1"/>
<display_value value=".BarBrawling8CompleteCombo"/>
<effect_group>
<requirement name="CVarCompare" cvar=".BarBrawling8CompleteCombo" operation="GTE" value="6"/>
<triggered_effect trigger="onSelfBuffStack" action="AddBuff" buff="buffPerkBarBrawling8CompleteBonus"/>
<triggered_effect trigger="onSelfBuffStack" action="RemoveBuff" buff="buffPerkBarBrawling8Complete"/>
</effect_group>
<effect_group name="cleanup">
<triggered_effect trigger="onSelfBuffRemove" action="ModifyCVar" cvar=".BarBrawling8CompleteCombo" operation="set" value="0"/>
</effect_group>
</buff>
<buff name="buffPerkBarBrawling8CompleteBonus" name_key="perkBarBrawling8CompleteDesc" description_key="perkBarBrawling8CompleteLongDesc" icon="ui_game_symbol_7th_curse" icon_color="0,255,0" icon_blink="true">
<stack_type value="ignore"/>
<duration value="7"/>
<effect_group name="Cleanup">
<triggered_effect trigger="onSelfPrimaryActionEnd" action="ModifyCVar" cvar=".BarBrawling8CompleteBonusRemove" operation="add" value="1"/>
<triggered_effect trigger="onSelfSecondaryActionEnd" action="ModifyCVar" cvar=".BarBrawling8CompleteBonusRemove" operation="add" value="1"/>
<triggered_effect trigger="onSelfBuffRemove" action="ModifyCVar" cvar=".BarBrawling8CompleteBonusRemove" operation="set" value="0"/>
<passive_effect name="EntityDamage" operation="perc_add" value="0,0,2,2" duration="0,.08,.09,777">
<requirement name="ItemHasTags" tags="perkBrawler"/>
</passive_effect>
</effect_group>
<effect_group name="play crunchy sound">
<requirement name="ItemHasTags" tags="perkBrawler"/>
<requirement name="CVarCompare" cvar=".BarBrawling8CompleteBonusRemove" operation="GT" value="0"/>
<triggered_effect trigger="onSelfAttackedOther" action="PlaySound" sound="breakleg"/>
</effect_group>
<effect_group name="wait until the next attack (with any weapon) has started AND ended, then remove bonus">
<requirement name="CVarCompare" cvar=".BarBrawling8CompleteBonusRemove" operation="GT" value="1"/>
<triggered_effect trigger="onSelfPrimaryActionEnd" action="RemoveBuff" buff="buffPerkBarBrawling8CompleteBonus"/>
<triggered_effect trigger="onSelfSecondaryActionEnd" action="RemoveBuff" buff="buffPerkBarBrawling8CompleteBonus"/>
<triggered_effect trigger="onSelfAttackedOther" action="PlaySound" sound="breakleg"/>
</effect_group>
</buff>
EDIT: Added another chunk of code.