They repeat roughly every year. 1,000,000 periods of 30 seconds is just less than 350 days.
Rather pleasingly, 30,000,000 seconds is roughly 1 year.
So, assuming a uniform distribution of codes, you would expect to see a repeat after a year.
You also need to keep in mind the birthday paradox. Repeated codes will happen with high probability after around sqrt(1,000,000) = 1000 code emissions. https://en.wikipedia.org/wiki/Birthday_problem