You do not need to worry about having to fly one international trip at least once in an 18-month period to maintain your international qualification. United has announced that this policy, "has not yet been implemented." However, should this restriction be enforced in the future, United has stated that they will provide "advanced notice of this provision's implementation."
Hopefully they will never decide on this 18-month period restriction. It's a bit silly anyway, because you can have a F/A that works difficult Premium Transcons (LAX-JFK) for 18 months straight, and then cannot work an LAX-MEX trip because he/she is not considered 'qualified' to work 3 1/2 international hours on an Airbus!
So... Shuttle away, hang out in Boise or Wichita or Orlando, and charge coach passengers for liquor for as long as you want...cause you still remain internationally qualified no matter how long you fly domestic...at least for now thank goodness.