Why would you want to buy the half fare card that much in advance?
There is no reason to buy it early: They are always avaliable and do not change in price. If you are talking about the monthly ‘Swiss Half Fare Card’ (Which is not the same as the yearly Halbtax!) The change in price by January 1st already has taken effect for Half Fare Cards valid next year, even if you were to buy it right now. I would assume that you can buy it at least 60 days in advance.
I recommend you not to buy it much before you need it so that you have no refund costs in the case that plans change.