i dont believe there is a general answer for this. this will really depend on the person.
generally a good "safe" dose is about 30mg ed. i personally use 40mg. the only way u will know for sure is to get a baseline of your values from a doctor, start off small such as 20mg (10 pre and 10 post workout) for 4 weeks, and get your values checked again. if everything is ok u could up it to 30mgs ed and do the same thing.