I'm finding this very tough to answer, the method of detection is critical in how far we can detect. There are two probable methods I can think of, one superior to the other. The first method involves the speed of light and our production of waves. The second involves how we have tailored our atmosphere.
Our production of waves (radio) started late 19th century, if we use a point of reference, say 1900; we have been broadcasting for 115 years, at light speed a species no further than 115 light years away could detect us. Hence the idea of the SETI program as Rahul has suggested, with the intention of broadcasting ourselves.
The best method, and the one I can see working for humans in their search for others, is atmospheric poisoning. There are specific hydrocarbons in our atmosphere which are thought to only be produced by man, if we think like this, then it's plausible we may also detect atmospheric poisoning around an exoplanet. Detecting oxygen simply is not enough, as it's not indicative that life exists, oxygen can be produced naturally in limited quantities as found elsewhere in the solar system however to sustain carbon based lifeforms such as ourselves there would have to be a major abundance. Detecting pollutants is the more logical way to conceive detection. If we are able to produce elements not naturally found, it is a clear indication that a species put it there. This also relies on the speed of light, however man made pollutants have existed pre-wave era, and have had longer to transmit light than our production of waves. The downside is detection method of the pollutants, currently as humans we rely on either using a star with a transiting planet to determine composition, or less precisely spectrum data (which doesn't indicate atmospheric material).
Another viewpoint is looking at the Kardashev scale, one could put forth that we have the technology to determine that answer based on energy consumption. If we could detect a massive gravitational field and no apparent source of energy, the energy may well be harvested by another species; such as a Dyson sphere. Such a detection I believe would be too easy to overlook as it's not something our species is actively searching for. While this holds truer for more of a theoretical detection, another species may be able to detect energy consumption on our planet, through lighting of our planet and atmosphere along with increasing surface temperatures.
I believe at best, as for human interference, we may be looking in the range of 100-150 light years. As for detection of life in general, I can't imagine pre-modern era if there was a simple way to determine that life existed if viewed from elsewhere apart from the fact we had a stable system containing liquid water and atmospheric oxygen.
We may be too reliant on providing the argument from our point of view being carbon based lifeforms, if another species advanced or more than us wasn't carbon based, it could very well be they are looking for other indications more localized to their own species, in the same way we look for indications which we imagine detecting ourselves with.
EDIT: As requested by Rob Jeffries; NO, using transit photometry using today's current technology is not yet possible. At
1ly Earth would appear as
2.776mas, which is possible by ESO's Very Large Telescope which has an angular resolution able to image in milliarcseconds. At
10ly Earth would appear as
277.6μas, possible after completion of the Cherenkov Telescope Array which has an angular resolution able to image in microarcseconds. Whilst the Cherenkov Telescope Array, is limited to
400nm and not able to image
1μas, at this next level it's we are imaging at
100ly. The Gaia spacecraft can resolve up to
20μas however is not able to image at this level. The NASA Ames Research Center is demonstrating resolution abilities down to
5μas at the attempt to resolve down to
1μas, however again that is not imaging resolution. For radio waves, truly enough I had not mentioned the inverse square law and wave degradation. For us as humans, yes a few light-years may be possible with a realm of possibility opening up with the Square Kilometer Array.
If you want me to retract my guesstimate from the first time, pollution and transit photometry are in fact possible using today's existing technology within
1ly, on par with existing radio receivers within
1yr. If you deter from the fact that the new instruments are however not built yet you can vastly increase this up to
100ly, just because something isn't built does not make the technology non-existent (Is the SKA technology feasible? Yes, we have the technology to make it happen right now, we just haven't done so. That does not make it technology that does not exist).
Seti Home has published finding of the the first earth sized planet detected from transiting. Further publication by Cornell University Library claims that the planet is within the habitable zone and implies it's within possibility of having an atmosphere and liquid H20 on it's surface. The Kepler spacecraft detected this finding, in case you are unaware, Kepler maps light curves as a body transits across the face of another body, this is called Transit. To even suggest that this technology doesn't already exist is preposterous, if you want a true analogue to Earth as is, with technology already existing;
1ly, if you want to use technology possible however not built;