Time limit for letters of introduction and or recommendation

[TSteven]

Is there such a thing as a "time limit" for a letter of introduction or for a letter of recommendation?

The situation that prompts this question is as follows.

I've got a cousin who is planning on attending college in the spring. Her sister is a legacy to one of the chapters on the campus.

My understanding is that the campus just completed formal rush and that not all the chapters made quota. So when my cousin enters college in the spring, there may be spots open with some of the chapters that either did not make quota or are not at total. On the other hand, it may be that by next semester, my cousin will not be able to rush if the chapters can not take any new members. Say the chapters COB and or make total by then. Or perhaps the chapters that do have spots available, are just not a fit for my cousin and or visa versa.

So if letters of introduction and letters of recommendation are written on her behalf, if need be, can they "carry over" to the fall and formal rush? Or will she need to secure recommendations again?