When it comes to FAQs and erratas my search-fu generally fails me.
Ive devised a truly devious wondrous item - a dust of summon swarm..activated by contact with saliva. Better than poison in some environments, but I've come across a tricky issue - how long will the swarm last? The original spell has a duration of "concentration+2 turns".
How does one handle concentration durations on magic items?
You've made a custom item. There are no rules for this. It's up to your GM to decide how it works. If you are the GM congratulations, you decide.
If you want advice, that is a different topic.
You might consider looking at this item for guidance.