It Expires
Your written will expire 24 months after you take it unless you are employed by a part 121 or part 135 airline. If you don't think you will have the minimum flight hours within 24 months, don't take it. If you do think you will have the minimum flight hours within 24 months, take it. Then, if you get to your 23rd month, go take the ATP checkride, even if you do it single engine in a 172.
If you are looking at it from a hiring standpoint, the only airline that will hire you right now (of which I am aware) with less than 1000 hours is Great Lakes, so unless you are planning to go there relatively soon, it won't matter much in the short term.