GEnerally the promotional credit should be applied every month... but the problem most people are having is that the credits are not being automatically applied. Hopefully the new promotions are applied more quickly then previous promotions.... but if you do get charged, you will just have to call up customer service to let them know the credit had not been applied... but i was told concerning the new free line promo, you should see the credits in your first bill.
Hey there vegas68, congrats on snagging a line before that promo ended! barcodeable is correct, the credits are applied automatically by the billing system, so when you review your plan on MyT-Mobile, it will still show the regular rate for the line. Although I certainly hope you won't, if at any point you do have trouble with automated bill credits, please Contact Us so that we can get you squared away. We don't want you to pay more than you should, ever.