It's not clear if you mean using SETI technology, or using ANY technolgy. If the latter, PH has already provided you with an answer. If the former, which is in effect the same as asking how far away can we or a similar civilization of similar technology detect the sorts of radio signals that are generated on earth by humans, the answer is somewhere beyond 1000 LY, and substantially more if you use something the size of the Arecibo dish for your antenna. And even more than that again if Alfred the Alien is using an Arecibo size antenna to transmit his signals!
I'm quoting RA for the 1000 LY number, so if it turns out wrong, blame him!
Thanks, 1000 LY radius is a LOT of stars!