Given that we have a dataset of one known technological civilization, it's hard to make any assumptions. However, if you look at our history of technological development, there may be a window of opportunity for detecting other civilizations. We began broadcasting radio waves only a little over 100 years ago. At first, they were low power and low frequency transmissions that would likely be lost in the cosmic background noise. By WWII, higher powered and higher frequency transmissions were being broadcast, especially by radar which adds a directional aspect, effectively increasing the radiated power. In subsequent years, we went through fairly high powered TV broadcasts but that seems to be tapering off. Today, an increasing percentage of broadcasts are being carried on cable or by largely downward directed satellite signals like DirectTV. Earlier broadcasts were also AM and later FM modulated, which is easy to detect. Today, a high percentage of RF emissions are pulse encoded and hard to distinguish from background noise. As we switch to digital TV broadcasts, this may be even more the case. Given current trends, it's possible that 100 years from now, the RF energy radiated from the Earth may be significantly less than it is today and virtually all of it would likely be digital, making it harder for someone else to detect us.

If other civilizations followed a similar technological evolution, we would have a narrow window of opportunity to detect them. If they were the equivalent of a couple hundred years behind us, we may no longer be listening when they begin to transmit. Likewise, we may no longer be transmitting when they're capable of listening. On the other hand, if they were even a couple hundred years more advanced than us, then we might have missed our window of opportunity to detect them. It's possible that the technological window of opportunity could be as short as a couple hundred years, a miniscule amount of time on a galactic scale.

It reminds me of an African Bushman like the one portrayed in "The Gods Must Be Crazy." (Excellent movie). The Bushman can live his life completely oblivious to the radio transmissions passing all around him. He would have no means to detect the transmissions. If someone told him that other people could talk to one another and even send pictures thousands of miles away, he wouldn't have any frame of reference to understand how such a thing could happen.