Rather than times per week, I'd concentrate on the total volume of work being applied and the quality of the exercises.
Power shrugs (the absolute best)
High Pulls
Cleans
Deadlifts
Traps respond far better to dynamic pulling (speed work i.e. olympic lifts) and compound exercises then they do in isolation (i.e. regular shurgs).
I know of no lifter who utilizes barbell or dumbell shrugs once they have sampled the power shrug or become proficient at the olympic lifts - no comparison for results.