Hi - the general advice is you go to your Gp after 12mths of trying, unless you are over the age of 35 in which case go after 6mths. But if you have had no periods at all it would be good to talk it over with your gp, unless you have already done so? Is this normal for you, to have no periods? Sometimes low weight can affect periods, otherwise other medical reasons so worth getting checked out.
Sorry if you have already been to gp - I can't think of anything else.