Assuming that space faring societies capable of interstellar travel are of non-hostile and likely of amiable disposition while taking into consideration that carbon or silicon based life is ubiquitous in the galaxy the spread of such species should actually be quite rapid once a civilization reaches maturity. A mature civilization should be at a technological or biological level of evolution where the home star’s energy is sufficient to power said civilization on the home planet in a sustainable manner. Natural tendency of species to propagate logically leads one to believe that the home planet will become a seed from which it expands throughout the solar system of origin and the surrounding neighborhood of the galaxy. At the present level of scientific knowledge, it is understood that using anti-matter propulsion relativistic speeds of about .8 the speed of light are naturally possible. Compared to the time frame of the evolution of any particular sentient space faring species, taking humans as an example, time needed to travel a distance of a 1000 parsecs would be .8 X 3.26 or about 2,608 years as observed from point of origin. One would assume that life span of a species at this point in development would be either technologically approaching immortality with redundant safety systems, or a similar regenerative biological approach. Compared to the approximately 4,000,000 years it took mankind to reach this cusp of biological and technological evolution said 2,608 years is a blink of an eye. This speaks strongly that sentient species based on current understanding of the planetary geology, evolutionary biology and physics should very rapidly propagate once point of maturity is reached. From this point of view let us examine how interstellar commerce in a galactic community would look taking into consideration current technological advancement of humanity.  Once habitable planets suitable for colonization are identified, the species could theoretically seed them in less then 3,000 years in a somewhat spherically shaped volume of space that exceeds the thickness of the galactic disk. It would take about 100,000 years to spread throughout the entire galaxy. Age of life on Earth (more than 4,000,000,000 years) as compared to the measly 4,000,000 years (about what Homininae needed to become space faring) indicates a 100,000 years to colonize the galaxy is for the lack of a better word: peanuts (.025 of the time it took men to evolve - assuming it could have happened at other branches of evolution; or another species of the animal kingdom, could have become an instant space faring hit out of the 542 million years of evolution or so: 0.000184 of the time it took for that to happen on Earth).

Anthropocentric hubris aside, the galaxy should be buzzing with interstellar trade, if it isn’t, this is how humans will make it so with the use of present technology and scientific understanding: interstellar quantum communication laser relay stations outside the heliopause would serve as basis for interstellar transmission of data or in other words digital commerce (or in more  rational, Utopian  terms not smelling of greed and need of profit: free information exchange). Since the presence of a sentient spacefaring species is required to receive such a transmission it is logical to assume that informational delivery (at the speed of light) would be sufficient to build (or print) whatever goods the information exchange calls for (the prerequisite for the presence of a relay station on the other hand, is previous robotic or sentient presence to position it in any given star system). Transportation of physical objects in interstellar exchange would be delivered by spacecraft powered with anti-matter propulsion (harvested, or generated at the point of origin star system) much like the original colonization efforts that seed habitable planets. Life span of interstellarly space faring sentient life forms is either unlimited or even as compared to present human span of 90 years: completely sufficient for the hops from start to star.

Project Eden: panspermia of the galactic neighborhood with humans. To streamline the process and make it less of a psychological burden on the colonists, one would have to A. eliminate the tediousness out of the trip and B. make them feel indigenous to the world they are to inhabit. The first can be done by either keeping the occupants of the colony spaceship busy or hibernating them for the trip. This complicates things in a number of ways: it requires a ship with a greater mass that can support the life forms and leaves one with the point B unresolved. The best way is to send frozen embryos with incubators and robots who will raise the new humans as indigenous to the planet of destination. Upon landing the spacecraft would of course serve as self enclosed habitat until the time when founders build a colony and start interstellar information and technology transfers (say nature films and analysis of the newly colonized planet’s geology followed by original creative content of the newly founded civilization etc.) A relay station would detach in the heliosphere upon arrival in the star system being colonized to insure up to date technological information from the home world is fed to the new colony. Once the newly colonized star system reaches maturity in the sense that it can independently develop and undertake subsequent colonization efforts it serves as a linking point in the galactic internet of worlds and a stage for further galactic exploration.

This approach to panspermia has a number of other benefits: in the event of mission failure during flight no human life is lost. Upon discovery of a sentient species on the planet of destination that doesn’t welcome new inhabitants: the mission can be either aborted or changed to a purely scientifically observational one and/or  a communication link with an existing civilization can be established. However one can assume that the detection of a sentient civilization will be possible prior to colonization so appropriate approach can be planned for prior to launch.

P.S. I still contend that instantaneous space-time independent communication is viable and physically possible.