If you are looking to get retweeted and nobody picks your tweet up within the first hour, chances are that nobody ever will. Only 1.63% of all retweets happen in the second hour and a minuscule 0.94% in the third hour. The same is true for @replies, too; 97% of all replies happen within the first hour.
I can attest to this. The things I’ve made that have BLOWN up have ignited within the first couple minutes. Very few delayed reactions. Either it sings, or it doesn’t.
This sort of shelf life is similar in Facebook—but what’s interesting in twitter is that the community tolerates a TON of content before pushing back, so sometimes people that drive a ton of RT’s are also driving a lot of volume.