I've not personally looked into any studies for blue about this, but when TTC when I got my son we were doing everything we could to optimise our chances of getting pregnant (not swaying) and I read back then that every 36hrs was optimum if you had age on your side. That ensures that the most sperm will make it in there. Maybe there is a study somewhere that says that every 3-4 days is best, but I haven't read it and logically I can't see why this would be.