It isn't possible to predict the duration of therapy, it depends on the issues you want to work 

on and how you define success. In general, a specific issue with a short history will probably be dealt with fairly quickly, for example in 10 to 20 sessions of counselling. However, a less specific issue with a long history such as "I always seem to choose the wrong partner" will probably need a much longer time period to be dealt with, maybe six months to several years.