e-Juice is available in many strengths including 24mg/ml, 18mg/ml, 11mg/ml, etc.  Many people want to get a replacement which is similar to that of their cigarette counterpart.  For instance, someone may smoke an ultralight brand x.  There is no accurate method to determine what you will need other than trial and error.  You can ask people and they will tell you what strength they require based on personal experience. 

From my own personal experience, and discussing this topic with others, this is my conclusion:

For full flavored regular smokers, 18mg/ml is a good starting point.  If it happens to be too strong, you can always dilute it with propylene glycol (PG) or vegetable glycerine (VG).  However, if you do not feel enough satisfaction from it, you cannot easily add nicotine content to it.  So save this bottle for later.

For light smokers, 11mg or so is a good starting point.  Again, if it happens to be too strong, you can always dilute it.

For ultralight smokers, try going with the lowest possible strength, this may be about 4mg/ml.