Cardamom (Elaichi)

I have in mind brewing (I generally brew from extract + grain) a batch of sweet stout and add some Cardamom (Elaichi) to flavor it. Does anybody has some experience with it and/or suggestions (what amount for a 20L batch, when to add it etc.) and or tips. ???

I've used a tiny amount with other herbs in a wit, where the herbal mix is supposed to be in the background. 

It's hard to say how the dark grains of a stout will partially mask or obliterate such herbs.  Still, you may want to aim low on the first try, adding it in the final minutes so all the aroma does not get boiled off.  If it gets covered up, you could make a cardamon tea and dose the batch at transfer to secondary or bottling.

+1 for adding it at bottling

many thanks.  I hesitate between adding some 30-50 gr to the last 5 minutes of the boiling or to "dry hop" the same amount or may be both. what do you think ?

I'd definitely try it at end-of-boil first, and then see if you taste it later in the processing.