# How fast is the Cosmic Microwave Background Radiation (CMBR) changing?

I know that the Cosmic Microwave Background Radiation (CMBR) is the leftover radiation from the "surface of last scattering".

However, at every instant the surface is changing (at the rate of flow of time). So how constant is the CMBR?

Just on dimensional grounds I would expect that $$\text{angular size of fluctuation in radians} \times \text{time since recombination}$$ is the time scale for significant changes in the CMB (where you choose what makes a fluctuation "significant" and select the angular size on that basis).