[uxcn]

Two civs probably need to be within half a millennium or so of technological development (human time) to have any possibility of communicating.

Intelligence seems to expand polynomially or exponentially by a lot of measures though (e.g. technology). The closest analogy for us compared to another civilization that could actually receive our radio communication I think would be chimpanzees using sticks to eat termites.

If I had to guess, I'd guess there's probably some kind of technological asymptote, but considering the scale of the universe (more importantly its complexity) I think we are probably nowhere near it.