Monthly. Until they start charging some sort of fee for doing so, it's an interest free loan. The rewards dollars aren't, to me, worth it.
What I typically do is figure out (based on that years rate) what a year of dues is going to cost, take it out of my tax refund, and throw it in our savings account. I then transfer the amount out of savings, into checking, every month....and whatever we earn in interest just continues to sit there and earn interest, year after year.