Without any other information, it is reasonable to place the average to whatever time it takes us (probably a bit over a century), but I wouldn't put a lot of confidence in that figure, having been obtained from a single data point. Radio visibility could conceivably range from a mere decade (consider that computers could have been developed before radio -- had Babbage been more successful -- and expedite technological advances) to perhaps millennia (consider dim-witted beings that live for centuries and do everything we do ten times slower).

Several different organizational schemes might also be viable for life and lead to very different time tables: picture a whole ant colony as a sentient being, for instance (ants being akin to neurons). Such beings would be inherently less mobile than humans. That may skew their technological priorities in such a way that they develop short range radio before they even expand out of their native island, in which case their radio visibility window would be nil because by the time they have an use to long range communication, they would already have the technology to do it optimally.

Furthermore, an "ant neuron" is possibly a lot more sophisticated than each neuron in our brain, but also much slower, so an "ant brain" might be the kind of slow, "dim-witted" intelligence that would go through the same technological steps orders of magnitude slower than we do while retaining very high resiliency and competitiveness.