There are many classes whose have different buffs in skills with different timers, some classes can buff themselves and party members for a long long time (example: Valkyrie who can buff accuracy to herself during 60s and that apply to party members too, and cooldown is 40s), however most of the classes has only 5s to 10s buff timing, sometimes 15s to 20s buff timing and cooldown are not always "coherence" between the duration of the buff and cooldown. I´m gonna put some skills from different classes just as an example:

♦ Valkyrie:  Heaven´s Echo:             Timing of the buff (20 DP and 15% accuracy) 60s duration and cooldown 40s

♦ Ranger:  Call of the Earth:            Timing  of the buff (9% Accu) 20s duration and cooldown 60s

♦ Sorceress:Shard´s of Darkness:     Timing of the buff (20 AP) 20s duration and cooldown 0s

♦ Drakania: Legacy:                        Timing  of the buff (20 AP & 15% atk speed Hexeblood  or  25 AP & 20 DP Dragonblood) 60s duration and cooldown 0s

I would suggest buff & add-ons should give you 20s & 15s duration and cooldown of the skills should be less than that, not overlaping the cooldowns with the duration timing.