The kit may become less effective over time; it is recommended to wait no more than three months before sending it in, though why wait? The sooner you have your results, the sooner you can jump on the fast-track to a healthier you!
In case you were interested, after collection, your sample can last several months (possibly up to a year) before the DNA starts to degrade (not that this is very important to you, as your sample is destroyed at the lab after your results are produced).