It is best to conduct your One On Ones on a weekly basis. The simplest reason for this is that you probably think about your work life in weekly increments. (According to research, it's roughly a three- to five-day window). You think about deadlines that are coming up this week. You tend to put off things that are due next week, even if they will take you several hours of work. You probably know what your schedule is this week, and maybe you know a bit of what your schedule is next week, but for the week after next, you have little sense of what your week will be like, in most cases.