What is a good amount of time to wait to check up on a recommender after the initial request?

Related question: Email to a professor after long time for recommendation

So, about a month ago, I sent three requests for letters of recommendation. Since then, two of them have finished their letters and one (I imagine) is quite busy and told me he probably won't be able to get to the letter for another two weeks [about a week prior to the application deadline], and the application is due on January 1 [which is a bit less than a month from now].

Would it be wise to send a reminder e-mail to him, and if so, how long should I wait?

[Please feel free to edit the title; I realize it's not very clear.]