The FAQ states
The various options are $10, $20, $30, $40 all valid for a period of 1 month from the date of purchase.
(The $ have now been extended to $50, $100 and $200, as well which needs to be added to the FAQ).

So once you buy a subscription, its valid for 1 month. If you have already posted the questions you needed to post and do not have any further questions, you can redeem the balance $. But you will get back only $75. Hence, its prudent if the user concerned buys the required number of $ at the outset. If he does fall short, he can buy more. I just stated this because abousetta stated about the $1000.

However, if you are the person answering the questions and you accumulate points over time, there is no validity for you. You can redeem the points once you reach a minimum of 100 points.