SMwutches has it on point. Free month of Xbox Gold Live supscriptions are for new accounts only. In addition, the console provides the Xbox Gold Live subscription and can recognize when a total of three (1) month trials have been given out and will cease to offer that promotion to any new profile beyond that point. This is important to know as if you purchase a console second hand that owner may have used up those accounts already on that one console.
As for promotions you find on the Xbox dashboard (now called the homepage) is sent out directly to that one specific account, you may not or will not see that same offer on another account downloaded to the same console as the promotion is sent out specifically for that specific account. These promotions can last as little as 24 hours up to how ever long the promotion is running. These offers are considerd, dashboard (homepage) promotions only and are must be accepted when presented and are not available via the website or phone support.
Passes that come included in your game, such as 48 hour gold trials are designed to be specifically for that game that they came with. Most often they will give you Gold Live whether or not you have used up your free trial before, or had a Gold Live subscription on your account in the past and are now a Free Live member. The trade off is it will only work for that specific game and you may find many other Gold Live subscription features locked or not accessable.