Classically, there would be no hard limit. In a perfectly silent, classical universe, you could imagine a device to detect the wave. But practically, the strength would eventually decay below the ability of your device to discriminate it from other sources of noise (including those from within the device).
In QM, any detector has a lower and lower probability of interacting with any photons from the source as the distance increases. A very skilled detector might pick up a photon now and then, but it would have to discriminate those from detections of noise and other sources.
There is no "edge of the universe" for such a wave to bounce back from. In most cosmological models, the energy of the wave continues into ever larger volumes of space, reducing the strength and ability to be detected. In some possible models, space has a large-scale curvature. It would be possible in those universes for the waves to eventually be concentrated into a smaller region, increasing the strength. This would be similar to creating a splash on a perfect water world. Waves would propagate from the splash and move outward, decreasing in strength. But if they were not attenuated, they could continue to the other side of the planet, where they would be concentrated once again at the exact opposite side.