Well, I'd imagine a lot of non-outcast visitors to the planet to wear oxygen masks or protective equipment to prevent contamination. Individuals such as members of the Golden Chrysanthemums or Lane Hackers or other factions who aren't biologically dependant on Cardamine, or visiting traders or freelance pilots would likely be careful not to expose themselves to the cardamine in the air, and you'd expect some visitor-only areas which are free of cardamine.
The timeframe for the complete cardamine dependancy for individuals visiting Malta would depend on a number of factors. Individuals with no prior exposure just breathing the air would take longer then individuals with some previous cardamine use (but aren't already dependant), like GC or Rogues who are visiting. As well, it depends on if contaminated food or drink is taken while on planet.
Timeframes range from 2 or so weeks downwards, I guess.