you really have to answer that yourself. try training them once a week, and if they do not respond you know you need to train them up to 2x a week. I say start with one time a week, because that way you are sure your not overtraining to start and not preventing growth if it is possible at once a week.
I train them once a week with shoulders. It works well for me. Traps also get a decent workout with heavy deadlifts sometimes, that is somewhat of a workout. lol just to keep in mind.
traps were a weak point for me but then I prioritized them and they got bigger. In my schedule of lifting I don't have planned days off, I take em' when I need them. So I'm constantly lifting and throughout the week I'll throw in weakpoint training all by itself and work the specific bodyparts that are lagging on me. Traps for a time was one of them so I ended up doing them twice a week. But really it varied..once they healed from each workout I would hit them again..It just so happened to average out to twice a week..more or less.
You need to be progressively lifting heavier weights. If you can recover in 3 to 4 days then train them twice a week. If you are not lifting heavier add more rest days.
My traps got the biggest when I was pulling heavy deads everyweek.
Do you feel like you could handle training them twice a week? If yes do it. Overtraining is not the same for everyone. I train chest twice a week at times and once at other times. It all depends on how you feel. Train as often as your body allows.
__________________ Best Competition Lifts
Squat: N/A
Bench: 500 IPA world Record (raw/pro) 655 shirted
Deadlift: 505 suited