To change the duration you'll have to edit the skill entry in the _RefSkill table. Have a look at all the SPEED skills and you will see that the duration is stored in Param4, change it to the value you want (e. g. 3600000) and don't forget to restart your GS afterwards. Then also edit the entry in your skilldata_xxxx.txt and encrypt it, so you'll also get the edited version of skilldata_xxxxenc.txt
If you want to change the description of a skill (I guess you're referring to the icon tooltip description next to the character HP/MP bar) you have to edit the UI_SkillName and UI_SkillToolTip_Desc column in _RefSkill and change the corresponding entry in your textdata_object.txt
Like this for example:
1 SN_ITEM_MALL_MOVE_SPEED_UP_100 이동 속도 증가 100% 0 0 0 0 0 Increase Moving speed 100% T¨ng tèc ®é di chuyÓn 100% 0 0 0 0 0 0
1 SN_ITEM_MALL_MOVE_SPEED_UP_100_TT_DESC 60분 동안 이동속도가 100% 증가 합니다. 0 0 0 0 0 Moving speed will increase 100% for 3600 sec. Tèc ®é di chuyÓn sÏ t¨ng 100% trong vßng 3600s 0 0 0 0 0 0