How addicted is a person that does a half gram of heroin a day?

It totally depends on how much he/she is used to taking, as tolerences vary from person to person. Although, usually as tolerence increases, the amount they would require to be 'satisfied' would also increase.

I have know some people who could make a half gram last days and others who could do 2-3 grams a day. So it does all depend on the individual, but continual use will cause a persons habit to increase, which is also why people move from snorting or smoking heroin to shooting it.