every four or five days even if you don't change anything in it. About four days after you've uploaded your resume you'll stop receiving calls from recruiters. This is not a good thing unless you really do want a little break from the employment search. There was a couple of times I did that on purpose too the last few years.
By uploading your resume so often it brings it to the top of the stack so to speak. The freshest resume gets the call in just about every case. While other sites have the ability for you to post your resume the primary ones I received calls in relation to were Monster and CareerBuilder. All the other sites (and there were several) just make it easier to apply to jobs. Beyond the resume I will mention that any cover letters you post need to be extremely general in nature. Usually there's the ability to modify it slightly so that you can focus it to the position.