Those who do not have a robust immune system, such as those with HIV, cancer, or autoimmune disease, are susceptible to developing active TB, which is contagious and can be deadly.
The conversion from exposure to active TB takes years. In those who never fight the disease successfully (i.e. those without a strong immune system at the time they are infected) have what is referred to as primary TB. It takes up to three years to develop primary TB, not days to weeks like most infections.
Latent TB can become active at any time throughout life if an infected individual becomes susceptible, such as if they develop cancer or get HIV. At that point it would be contagious.
Now, as far as your trip to Cambodia is concerned, if you may be exposed to those with active TB, you should wear a special mask which is fitted for your face and specially made to keep out TB. Contact the health department or the organizers of your trip to find out how to obtain one.
When you get home, annual TB testing required by schools should pick up a latent infection which could be treated (eradicated from your body) to prevent you from developing active, contagious disease later on.
I am not aware of any specific guidelines as to when to test, but three to six months after you return should be enough time to have converted to a positive test in those infected. As long as you do not have active disease, you will not be putting others at risk.