How quickly can I become addicted to a drug?

Just one use is enou. Some people get addicted just by experimenting while some don't depends on the response from brain - so best is to say no to experimenting drugs. I saw a girl who used stimulant once and developed seizures lost her balance now have poor memory - so once is also dangerous.
Soon. As quickly as you experience negative consequences. Consider attending a meeting of narcotics anonymous.