If you dont have in your config to set buff time, then 
go here: gameserver\data\stats\skills 
open file which you want to change buff file. 
  
For example: i want change Greater Might buff time his id is 1388 so i opened 1300-1399.xml with nutepad 
  
 
<skill id="1388" levels="3" name="Greater Might">
<!-- Done by KID -->
  <table name="#pAtk"> 1.04 1.07 1.10 </table>
  <table name="#mpConsume"> 54 62 69 </table>
  <table name="#spiritOre"> 1 2 3 </table>
  <set name="itemConsumeId" val="3031"/>
  <set name="itemConsumeCount" val="#spiritOre"/>
  <set name="mpConsume" val="#mpConsume"/>
  <set name="power" val="0.0"/>
  <set name="reuseDelay" val="6000"/>
  <set name="hitTime" val="4000"/>
  <set name="target" val="TARGET_ONE"/>
  <set name="skillType" val="BUFF"/>
  <set name="isMagic" val="true"/>
  <set name="operateType" val="OP_ACTIVE"/>
  <set name="castRange" val="400"/>
  <set name="effectRange" val="900"/>
  <for>
    <effect count="1" name="Buff" time="1200" val="0" stackOrder="0" stackType="Great">
      <mul order="0x30" stat="pAtk" val="#pAtk"/>
    </effect>
    <effect count="1" name="Grow" time="1200" val="0"/>
  </for>
</skill>
 
  
and where is <effect count="1" name="Buff" time="1200" val="0" stackOrder="0" stackType="Great"> 
change here in seconds: time="1200" 
300 - 5min 
1200 - 20min 
3600 - 1hour 
7200 - 2hours