At a given age of the Universe, which we will call t, the distance to the event horizon (Deh(t)) is given by the following expression
tmax is the maximum age of the Universe. This is infinity for the standard model of the Universe (known as the lambda cold dark matter model). However, there are other models of the Universe in which it has a finite maximum age.
a(t) is the comic scale factor. It measures the size of the Universe and is discussed in detail in my post https://explainingscience.org/2021/12/01/measuring-distances-in-cosmology/
c is the speed of light in a vacuum
The following diagram shows how the event horizon changes over over time in the standard model. In this model: 68% of the mass of the Universe is made up of dark energy, which drives the expansion of the Universe at an increasing rate, 27% dark matter and 5% ordinary matter.