How does the power emitted during thermal radiation depend on time (if it does)?
What are some sources referring to the particular relation between the power emitted during thermal radiation and time? How would the plot look like?
What are the fundamental underlying principles governing the derivation of an expression regarding the power under consideration?
The power emitted by a "black body" (or a "gray body") at constant temperature $T$ is given by the Stefan-Boltzmann law:
$$
P_{\text emit} = \alpha T^4\;,
$$
where $\alpha$ is a constant.
In the derivation of the above law, the temperature is assumed constant. This can be taken to mean that the radiating body is so large that the loss of energy due to radiation does not affect its temperature.
However, you want to understand how the temperature changes with time. In that case, it may be reasonable to assume the body undergoes radiative cooling and transfers heat to its environment, and thus cools (assuming the specific heat is positive, which is not necessarily the case on an astronomical scale).
The black or grey body not only emits thermal radiation, but it can also absorb thermal radiation from its environment. So, the total change in energy of the body can be approximated as:
$$
\frac{\delta E}{\delta t} = P = P_{\text emit}- P_{\text absorb} = \alpha (T^4 - T_{\text env}^4)\;,
$$
If we next assume that it is appropriate to use a constant specific heat, we can re-write this equation as:
$$
\frac{\delta E}{\delta T}\frac{\delta T}{\delta t}
\approx cM\frac{\delta T}{\delta t}
= \alpha (T^4 - T_{\text env}^4)\;,
$$
where $c$ is the specific heat and $M$ is the mass of the body.
If we assume the mass and the specific heat remain constant we can rewrite this as:
$$
\frac{\delta T}{\delta t}\approx \beta (T^4 - T_{\text env}^4)\;,\tag{1}
$$
where $\beta$ is a different constant (and is not related to the usual use of the symbol $\beta$ in statistical mechanics).
Please note that this derivation of Eq. (1) relied on many different assumptions, any of which may fail to be true in practice.
A further assumption that has not yet been stated explicitly is that the heat transfer is assumed to be entirely radiative. If conductive heat transfer is allowed then the rate of change of the temperature with time will have a different form, approximated by Newton's Law of Cooling.