Usually, a person will require 6-8 treatment sessions 6-8 weeks apart to obtain an 85-90% removal rate. Your hair has different growth cycles, and the laser targets the hairs in the Anagen stage of growth. Only 20% of your hair growth is in the Anagen stage at a time so you will need to get multiple treatments in order remove more hair and have more effective results. 
You may think that after only one or two sessions your hair is gone but if you leave it, the hair will continue to grow and some people think that the hair appears thicker. In reality it isn't and the hair will be removed with subsequent treatments. Think of hair removal like taking antibiotics, you need to complete all 6-8 treatments for optimal results.
You will notice a reduction of hair after your first treatment.